Patch version 3: simpler and shorter patch

* _PyObject_FastCall() keeps its kwargs parameter, but it must always be NULL. Support for keyword arguments will be added later.
* I removed PyObject_CallNoArg() and PyObject_CallArg1()
* I moved _PyStack_AsTuple() to Objects/abstract.c. A temporary home until the API grows until to require its own file (Python/pystack.c).

I also pushed some changes unrelated to fastcall in Python/ceval.c to simplify the patch.

Very few functions are modified (directly or indirectly) to use _PyObject_FastCall():

- PyEval_CallObjectWithKeywords()
- PyObject_CallFunction()
- PyObject_CallMethod()
- _PyObject_CallMethodId()

Much more will come in following patches.