Compile time check to assert # keywords == arity

[dyollb]

Experience has shown that C++ signatures may change over time: often arguments are added. Unfortunately, boost python will not complain if developers forget to update the bp export code.

make_function_aux in make_function.hpp already contains a check that the user did not provide too many keywords.
It would be useful if we could enable a check to assert that the number of keywords always is exactly equal to the number of arguments.