* Expand string_view support to str, bytes, memoryview
1. Allows constructing a str or bytes implicitly from a string_view;
this is essentially a small shortcut allowing a caller to write
`py::bytes{sv}` rather than `py::bytes{sv.data(), sv.size()}`.
2. Allows implicit conversion *to* string_view from py::bytes -- this
saves a fair bit more as currently there is no simple way to get such
a view of the bytes without copying it (or resorting to Python API
calls).
(This is not done for `str` because when the str contains unicode we
have to allocate to a temporary and so there might not be some string
data we can properly view without owning.)
3. Allows `memoryview::from_memory` to accept a string_view. As with
the other from_memory calls, it's entirely your responsibility to
keep it alive.
This also required moving the string_view availability detection into
detail/common.h because this PR needs it in pytypes.h, which is higher
up the include chain than cast.h where it was being detected currently.
* Move string_view include to pytypes.h
* CI-testing a fix for the "ambiguous conversion" issue.
This change is known to fix the `tensorflow::tstring` issue reported under https://github.com/pybind/pybind11/pull/3521#issuecomment-985100965
TODO: Minimal reproducer for the `tensorflow::tstring` issue.
* Make clang-tidy happy (hopefully).
* Adding minimal reproducer for the `tensorflow::tstring` issue.
Error without the enable_if trick:
```
/usr/local/google/home/rwgk/forked/pybind11/tests/test_builtin_casters.cpp:169:16: error: ambiguous conversion for functional-style cast from 'TypeWithBothOperatorStringAndStringView' to 'py::bytes'
return py::bytes(TypeWithBothOperatorStringAndStringView());
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../pytypes.h:1174:5: note: candidate constructor
bytes(const std::string &s) : bytes(s.data(), s.size()) { }
^
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../pytypes.h:1191:5: note: candidate constructor
bytes(std::string_view s) : bytes(s.data(), s.size()) { }
^
```
* Adding missing NOLINTNEXTLINE
* Also apply ambiguous conversion workaround to str()
Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>
