bpo-35134: Add Include/cpython/bytesobject.h file (GH-18494)

Add Include/cpython/bytearrayobject.h and
Include/cpython/bytesobject.h header files.

Move CPython C API from Include/bytesobject.h into a new
Include/cpython/bytesobject.h header file which is included by
Include/bytesobject.h. Do a similar change for
Include/bytearrayobject.h.