Avoid variadic functions

[encukou]

Avoid variadic functions. The feature isn't available in all languages that want to use the C-API.

(Per #4, these are fine as alternatives to functions with a fixed number of arguments.)

[pitrou]

This guideline is too strict IMHO. Rather, it should be:

For every variadic function in the C API, there should be a corresponding non-variadic function.

For example PyObject *PyTuple_Pack(Py_ssize_t n, ...) should also have a counterpart PyObject *PyTuple_PackArray(Py_ssize_t n, PyObject**).

[encukou]

Yes, this guideline relies on the general exception mechanism in #4.
(IMO, if PyTuple_Pack was being added today, it should be named PyTuple_VaPack so that the variadic-ness is exposed in the name.)