* tests: New test for ctypes buffers (pybind#2502)
* fix: fix buffer_info segfault on views with no stride (pybind11#2502)
* Explicit conversions in buffer_info to make clang happy (pybind#2502)
* Another explicit cast in buffer_info constructor for clang (pybind#2502)
* Simpler implementation of buffer_info constructor from Py_buffer.
* Move test_ctypes_buffer into test_buffers
* Comment on why view->strides may be NULL (and fix some whitespace)
* Use c_strides() instead of zero when view->strides is NULL.
c_strides and f_strides are moved from numpy.h (py::array)
to buffer_info.h (py::detail) so they can be used from the
buffer_info Py_buffer constructor.
* Increase ctypes buffer test coverage in test_buffers.
* Split ctypes tests and skip one which is broken in PyPy2.
