fuzz: extract some common utilities and use modern C++ idioms Alex Gaynor suggested we should probably copy the left and right sides of diffs to new blocks so we can detect over-reads in the diffing code, and I agree. Once I got into that, I realized we should do things with C++17 idioms rather than keep using malloc() and free(). This change is the result. I tried to split it more than this and failed. Everything still compiles and works in the oss-fuzz container, so I think we can count on C++17 being available! Differential Revision: https://phab.mercurial-scm.org/D3675