Move forward declarations of Python C API types to a new pytypedefs.h
header file to solve interdependency issues between header files.
pytypedefs.h contains forward declarations of the following types:
* PyCodeObject
* PyFrameObject
* PyGetSetDef
* PyInterpreterState
* PyLongObject
* PyMemberDef
* PyMethodDef
* PyModuleDef
* PyObject
* PyThreadState
* PyTypeObject
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.
