bpo-29548: Recommend PyObject_Call APIs over PyEval_Call APIs. (GH-75)

PyEval_Call* APIs are not documented and they doesn't respect PY_SSIZE_T_CLEAN.
So add comment block which recommends PyObject_Call* APIs to ceval.h.

This commit also changes PyEval_CallMethod and PyEval_CallFunction
implementation same to PyObject_CallMethod and PyObject_CallFunction
to reduce future maintenance cost.  Optimization to avoid temporary
tuple are copied too.

PyEval_CallFunction(callable, "i", (int)i) now calls callable(i) instead of
raising TypeError.  But accepting this edge case is backward compatible.