Pass the current interpreter (interp) rather than the current Python
thread state (tstate) to internal functions which only use the
interpreter.

Modified functions:

* _PyXXX_Fini() and _PyXXX_ClearFreeList() functions
* _PyEval_SignalAsyncExc(), make_pending_calls()
* _PySys_GetObject(), sys_set_object(), sys_set_object_id(), sys_set_object_str()
* should_audit(), set_flags_from_config(), make_flags()
* _PyAtExit_Call()
* init_stdio_encoding()
* etc.

The _Py_IsMainInterpreter() function now expects interp rather than
tstate.

Replace PyModule_AddObject() with PyModule_AddType() in the
_contextvars module (Python-ast.c).

Add also the module name to _contextvars types name.

Only clear _token_missing in the main interpreter.

Add get_xxx_state() function to factorize duplicated code.

In debug mode, ensure that free lists are no longer used after being
finalized. Set numfree to -1 in finalization functions
(eg. _PyList_Fini()), and then check that numfree is not equal to -1
before using a free list (e.g list_dealloc()).

Each interpreter now has its own context free list:

* Move context free list into PyInterpreterState.
* Add _Py_context_state structure.
* Add tstate parameter to _PyContext_ClearFreeList()
  and _PyContext_Fini().
* Pass tstate to clear_freelists().

Remove the following function from the C API:

* PyAsyncGen_ClearFreeLists()
* PyContext_ClearFreeList()
* PyDict_ClearFreeList()
* PyFloat_ClearFreeList()
* PyFrame_ClearFreeList()
* PyList_ClearFreeList()
* PySet_ClearFreeList()
* PyTuple_ClearFreeList()

Make these functions private, move them to the internal C API and
change their return type to void.

Call explicitly PyGC_Collect() to free all free lists.

Note: PySet_ClearFreeList() did nothing.

If only offsetof() is needed: include stddef.h instead.

When structmember.h is used, add a comment explaining that
PyMemberDef is used.