Add METH_STACKLESS to prevent future collisions.

the bare METH_FASTCALL be used for functions with positional-only
parameters.

Calling builtin method is at most 10% faster.

Issue #29259, #29263. methoddescr_call() creates a PyCFunction object, call it
and the destroy it. Add a new _PyMethodDef_RawFastCallDict() method to avoid
the temporary PyCFunction object.

Issue #27810: Add a new calling convention for C functions:

    PyObject* func(PyObject *self, PyObject **args,
                   Py_ssize_t nargs, PyObject *kwnames);

Where args is a C array of positional arguments followed by values of keyword
arguments. nargs is the number of positional arguments, kwnames are keys of
keyword arguments. kwnames can be NULL.

Use _PyCFunction_FastCallKeywords() in ceval.c: it allows to remove a lot of
code from ceval.c which was only used to call C functions.

Issue #27848: use Py_ssize_t rather than C int for the number of function
positional and keyword arguments.

Issue #27809:

* Rename _PyFunction_FastCall() to _PyFunction_FastCallDict()
* Rename _PyCFunction_FastCall() to _PyCFunction_FastCallDict()
*  _PyFunction_FastCallDict() now supports keyword arguments