This commit adds `error_already_set::matches()` convenience method to
check if the exception trapped by `error_already_set` matches a given
Python exception type. This will address #700 by providing a less
verbose way to check exceptions.

* Support raw string literals as input for py::eval
* Dedent only when needed

The constexpr static instances can cause linking failures if the
compiler doesn't optimize away the reference, as reported in #770.

There's no particularly nice way of fixing this in C++11/14: we can't
inline definitions to match the declaration aren't permitted for
non-templated static variables (C++17 *does* allows "inline" on
variables, but that obviously doesn't help us.)

One solution that could work around it is to add an extra inherited
subclass to `object`'s hierarchy, but that's a bit of a messy solution
and was decided against in #771 in favour of just deprecating (and
eventually dropping) the constexpr statics.

Fixes #770.

* Arch-indep CMake packaging

Since pybind11 is a header-only library, the CMake packaging does not have to carry any architecture specific checks. Without this patch, the detection of pybind11 will fail on 32-bit architectures if the project was built on a 64-bit machine and vice-versa. This fix is similar to what is applied to `Eigen` and other header-only C++ libraries.

* nicer py::capsule destructor mechanism
* added destructor-only version of capsule & tests
* added documentation for module destructors (fixes #733)

We no longer need a nightly build now that 5.7 is released (and the
current nightly 5.8 is failing).

The extends the previous unchecked support with the ability to
determine the dimensions at runtime.  This incurs a small performance
hit when used (versus the compile-time fixed alternative), but is still considerably
faster than the full checks on every call that happen with
`.at()`/`.mutable_at()`.

This adds bounds-unchecked access to arrays through a `a.unchecked<Type,
Dimensions>()` method.  (For `array_t<T>`, the `Type` template parameter
is omitted).  The mutable version (which requires the array have the
`writeable` flag) is available as `a.mutable_unchecked<...>()`.

Specifying the Dimensions as a template parameter allows storage of an
std::array; having the strides and sizes stored that way (as opposed to
storing a copy of the array's strides/shape pointers) allows the
compiler to make significant optimizations of the shape() method that it
can't make with a pointer; testing with nested loops of the form:

    for (size_t i0 = 0; i0 < r.shape(0); i0++)
        for (size_t i1 = 0; i1 < r.shape(1); i1++)
            ...
                r(i0, i1, ...) += 1;

over a 10 million element array gives around a 25% speedup (versus using
a pointer) for the 1D case, 33% for 2D, and runs more than twice as fast
with a 5D array.

Fixes #754.

This extends the trivial handling to support trivial handling for
Fortran-order arrays (i.e. column major): if inputs aren't all
C-contiguous, but *are* all F-contiguous, the resulting array will be
F-contiguous and we can do trivial processing.

For anything else (e.g. C-contiguous, or inputs requiring non-trivial
processing), the result is in (numpy-default) C-contiguous layout.

The only part of the vectorize code that actually needs c-contiguous is
the "trivial" broadcast; for non-trivial arguments, the code already
uses strides properly (and so handles C-style, F-style, neither, slices,
etc.)

This commit rewrites `broadcast` to additionally check for C-contiguous
storage, then takes off the `c_style` flag for the arguments, which
will keep the functionality more or less the same, except for no longer
requiring an array copy for non-c-contiguous input arrays.

Additionally, if we're given a singleton slice (e.g. a[0::4, 0::4] for a
4x4 or smaller array), we no longer fail triviality because the trivial
code path never actually uses the strides on a singleton.

Instead of a segfault. Fixes #751.

This covers the case of loading a custom holder from a default-holder
instance. Attempting to load one custom holder from a different custom
holder (i.e. not `std::unique_ptr`) yields undefined behavior, just as
#588 established for inheritance.

py::arg() doesn't only specify named arguments anymore, so the error
message was misleading (e.g. when using `py::arg().noconvert()` and
forgetting `py::arg()` for a second positional argument).

We now require (and enforce at compile time):
- GCC 4.8+
- clang 3.3+ (5.0+ for Apple's renumbered clang)
- MSVC 2015u3+
- ICC 15+

This also updates the versions listed in the README, and removes a
now-redundant MSVC version check.

This adds brief API documentation for make_iterator/make_key_iterator,
specifically mentioning that it requires InputIterators.

Closes #734.

[skip ci] (no code change here)

We can't support this for classes from imported modules (which is the
primary purpose of a ctor argument base class) because we *have* to
have both parent and derived to properly extract a multiple-inheritance
base class pointer from a derived class pointer.

We could support this for actual `class_<Base, ...> instances, but since
in that case the `Base` is already present in the code, it seems more
consistent to simply always require MI to go via template options.

This puts the fold expressions behind the feature macro instead of a
general C++17 macro.

It also adds a fold expression optimization to constexpr_sum (guarded
by the same feature macro).

Fixes #738

The current check for conformability fails when given a 2D, 1xN or Nx1
input to a row-major or column-major, respectively, Eigen::Ref, leading
to a copy-required state in the type_caster, but this later failed
because the copy was also non-conformable because it had the same shape
and strides (because a 1xN or Nx1 is both F and C contiguous).

In such cases we can safely ignore the stride on the "1" dimension since
it'll never be used: only the "N" dimension stride needs to match the
Eigen::Ref stride, which both fixes the non-conformable copy problem,
but also avoids a copy entirely as long as the "N" dimension has a
compatible stride.

Instead of relying on sometimes missing C++17 definitions

This adds VS 2017 to the build matrix, plus various other small
appveyor build changes:

- conda version bumped from 3.5 to 3.6
- build newer versions/architectures/python first (i.e. VS2017/x64/3.6
  is the first build, VS2015/x86/2.7 is the last)
- stop building after a job failure: often a build failure in one
  occurs everywhere; this just stops processing jobs (freeing them up
  for other PRs) if an error is hit.

Annoyingly, appveyor doesn't allow excluding tests: i.e. the test matrix
is always dense (appveyor issue 386), so for now we'll just run
everything.  (Once appveyor issue 386 is resolved, we can come back and
cut this down to 4-5 builds).

Fixes #731.

Generally, this applies to any caster made with PYBIND11_TYPE_CASTER().

Fixes #728.

Allows equivalent integral types and numpy dtypes

Allows use of vectors as python buffers, so for example they can be adopted without a copy by numpy.asarray
Allows faster conversion of buffers to vectors by copying instead of individually casting the elements

* Add value_type member alias to py::array_t (resolve #632)

* Use numpy scalar name in py::array_t function signatures (e.g. float32/64 instead of just float)

This won't affect much, but makes the code consistent with the
non-copying branch.

Fixes #697

The `decltype(...)` in the template parameter that gives us SFINAE
matching for a lambda makes MSVC 2017 ICE; this works around if by
changing the test to an explicit not-a-function-or-pointer test, which
seems to work everywhere.

RTD updated their build environment which broke the 1.8.14.dev build of
doxygen that we were using. The update also breaks the conda-forge build
of 1.8.13 (but that version has other issues).

Luckily, the RTD update did bring their doxygen version up to 1.8.11
which is enough to parse the C++11 code we need (ref qualifiers) and it
also avoids the segfault found in 1.8.13.

Since we're using the native doxygen, conda isn't required anymore and
we can simplify the RTD configuration.

[skip ci]