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
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