bpo-40421: Add Include/cpython/code.h header file (GH-19756)

bpo-35134, bpo-40421: Add Include/cpython/code.h header file.

code.h now defines PyCodeObject type in the limited C API. It is now
included by Python.h.

Give a name to the PyCodeObject structure: it is now called
"struct PyCodeObject". So it becomes possible to define PyCodeObject
as "struct PyCodeObject" in the limited C API without defining the
structure.