__module__ is the string name of the module the function was defined
in, just like __module__ of classes.  In some cases, particularly for
C functions, the __module__ may be None.

Change PyCFunction_New() from a function to a macro, but keep an
unused copy of the function around so that we don't change the binary
API.

Change pickle's save_global() to use whichmodule() if __module__ is
None, but add the __module__ logic to whichmodule() since it might be
used outside of pickle.