Enable keep/reject inputs from the corpus

[dacut]

This allows the fuzz target to indiciate whether an input was useful for the fuzz testing by returning Corpus::Keep or Corpus::Reject. Backwards compatibility is preserved by coercing the unit type () to Corpus::Keep.

This maps to 0 (Keep) and -1 (Reject) in the libFuzzer API: https://llvm.org/docs/LibFuzzer.html#rejecting-unwanted-inputs