fuzz: clean out most of fuzzutil It's now a header-only setup that just selects absl:: or std:: versions of things as needed, and a logging helper. There's some room for future cleanups here: we could move to just requiring a C++17 compiler and get rid of the absl stuff. Also, the mpatch parser has a fair amount of parsing the input string into char* blocks that we can and probably should fix that up to use FuzzedDataProvider as well. Differential Revision: https://phab.mercurial-scm.org/D7370