New PyFrame_GetBack() function: get the frame next outer frame.

Replace frame->f_back with PyFrame_GetBack(frame) in most code but
frameobject.c, ceval.c and genobject.c.

The PyThreadState_GetFrame() function now returns a strong reference
to the frame.

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.

Add frame_nslots() to factorize duplicate code.

PyFrame_GetCode(frame): return a borrowed reference to the frame
code.

Replace frame->f_code with PyFrame_GetCode(frame) in most code,
except in frameobject.c, genobject.c and ceval.c.

Also add PyFrame_GetLineNumber() to the limited C API.

This does not yet support the "long form" for arguments with type
comments.

We also totally skip `# type: ignore` comments, and there is no
support for func_type_input yet.

This commit also allows to pass flags to the new parser in all interfaces and fixes a bug in the parser generator that was causing to inline rules with actions, making them disappear.

If _PyCode_InitOpcache() fails in _PyEval_EvalFrameDefault(), use
"goto exit_eval_frame;" rather than "return NULL;" to exit the
function in a consistent state. For example, tstate->frame is now
reset properly.