PEP 489: Updates from Petr Viktorin.

Summary:

- PyModuleExport -> PyModuleDef (which brings us down to two slot types,
  create & exec)
- Removed "singleton modules"
- Stated that PyModule_Create, PyState_FindModule, PyState_AddModule,
  PyState_RemoveModule will not work on slots-based modules.
- Added a section on C-level callbacks
- Clarified that if PyModuleExport_* returns NULL, it's as if it wasn't
  defined (i.e. falls back to PyInit)
- Added API functions: PyModule_FromDefAndSpec, PyModule_ExecDef
- Added PyModule_AddMethods and PyModule_AddDocstring helpers
- Added PyMODEXPORT_FUNC macro for x-platform declarations of the export
  function
- Added summary of API changes
- Added example code for a backwards-compatible module
- Changed modules ported in the initial implementation to "array" and "xx*"
- Changed ImportErrors to SystemErrors in cases where the module is
  badly written (and to mirror what PyInit does now)
- Several typo fixes and clarifications