Rename Include/buffer.h header file to Include/pybuffer.h to avoid
conflicts with projects having an existing "buffer.h" header file.
* Incude pybuffer.h before object.h in Python.h.
* Remove #include "buffer.h" from Include/cpython/object.h.
* Add a forward declaration of the PyObject type in pybuffer.h to fix
an inter-dependency issue.