contrib: add some basic scaffolding for some fuzz test targets I'd like to get the majority of our C code covered by automated fuzz testing. I've started with bdiff because it was already decoupled from libpython and therefore was fairly quick to produce a working fuzzer. The code here is a little odd because I've been having trouble convincing libfuzzer to define a main and I threw in the towel. This code will also work with github.com/google/oss-fuzz, and once it lands in our main repo I intend to enable automated fuzzing in oss-fuzz with reports going to our security alias. Differential Revision: https://phab.mercurial-scm.org/D1875