Implement and test probability distributions used by WTF-PAD.

This project introduces the prob_distr.c subsystem which implements all the
probability distributions that WTF-PAD needs. It also adds unittests for all of
them.

Code and tests courtesy of Riastradh.

Co-authored-by: Taylor R Campbell <campbell+tor@mumble.net>
Co-authored-by: Mike Perry <mikeperry-git@torproject.org>