* Make sure specialized calls are also giving C functions SystemError in
case they return NULL.
* Also make sure we use call code that takes advantage of tuple size
knowledge for constant positional arguments.
* Check tuple size to catch errors in using the POSARGS tuple variants.
* Add specialization for keywords only and mixed calls for large gains
as dictionaries might be avoided.
* Merge CompiledCodeHelpers and CompiledFunction for better friendship
of code.
* For unknown called objects, use "tp_vectorcall" if available for 3.8
or higher.
* Pass around argument and keyword value vectors as constant values, as
these are not modified, might help compiler optimization.