To augment the various PyObject_CallMethod...() functions, we add _PyObject_VectorcallMethod() which uses the vectorcall calling convention. To be consistent with the other vectorcall functions, this function is provisional for now.

For efficiency, the "self" argument is NOT passed as separate argument but as part of the usual args array. In other words, the call looks like an unbound method call.

I am also adding _PyObject_VectorcallMethodId() taking a _Py_Identifier* as argument and convenience functions PyObject_CallMethodNoArgs() and _PyObject_CallMethodIdNoArgs(), analogous to the recently added PyObject_CallNoArgs().