The typenum for non-structured types is still accessible at ::value,
and the dtype object for all types is accessible at ::dtype().

The format strings that are known at compile time are now accessible
via both ::value and ::format(), and format strings for everything
else is accessible via ::format(). This makes it backwards compatible.

Add pybind11::make_key_iterator for map iteration

This allows exposing a dict-like interface to python code, allowing
iteration over keys via:

    for k in custommapping:
        ...

while still allowing iteration over pairs, so that you can also
implement 'dict.items()' functionality which returns a pair iterator,
allowing:

    for k, v in custommapping.items():
        ...

example-sequences-and-iterators is updated with a custom class providing
both types of iteration.

Improve constructor/destructor tracking

This commit rewrites the examples that look for constructor/destructor
calls to do so via static variable tracking rather than output parsing.

The added ConstructorStats class provides methods to keep track of
constructors and destructors, number of default/copy/move constructors,
and number of copy/move assignments.  It also provides a mechanism for
storing values (e.g. for value construction), and then allows all of
this to be checked at the end of a test by getting the statistics for a
C++ (or python mapping) class.

By not relying on the precise pattern of constructions/destructions,
but rather simply ensuring that every construction is matched with a
destruction on the same object, we ensure that everything that gets
created also gets destroyed as expected.

This replaces all of the various "std::cout << whatever" code in
constructors/destructors with
`print_created(this)`/`print_destroyed(this)`/etc. functions which
provide similar output, but now has a unified format across the
different examples, including a new ### prefix that makes mixed example
output and lifecycle events easier to distinguish.

With this change, relaxed mode is no longer needed, which enables
testing for proper destruction under MSVC, and under any other compiler
that generates code calling extra constructors, or optimizes away any
constructors.  GCC/clang are used as the baseline for move
constructors; the tests are adapted to allow more move constructors to
be evoked (but other types are constructors much have matching counts).

This commit also disables output buffering of tests, as the buffering
sometimes results in C++ output ending up in the middle of python
output (or vice versa), depending on the OS/python version.

Silence MSVC warning

PR #329 generates the following warning under MSVC:

    ...\cast.h(202): warning C4456: declaration of 'it' hides previous local declaration

This renames the second iterator to silence it.

Track registered instances that share a pointer address

reference_internal requires an `instance` field to track the returned
reference's parent, but that's just a duplication of what
keep_alive<0,1> does, so use a keep alive to do this to eliminate the
duplication.

It no longer applies since instances are now identified by both address
and type.

The pointer to the first member of a class instance is the same as the
pointer to instance itself; pybind11 has some workarounds for this to
not track registered instances that have a registered parent with the
same address.  This doesn't work everywhere, however: issue #328 is a
failure of this for a mutator operator which resolves its argument to
the parent rather than the child, as is needed in #328.

This commit resolves the issue (and restores tracking of same-address
instances) by changing registered_instances from an unordered_map to an
unordered_multimap that allows duplicate instances for the same pointer
to be recorded, then resolves these differences by checking the type of
each matched instance when looking up an instance.  (A
unordered_multimap seems cleaner for this than a unordered_map<list> or
similar because, the vast majority of the time, the instance will be
unique).

Move support for return values of called Python functions