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

Docs: make it explicit that we're ignoring the return value of the
function under test. Add comments from libfuzzer explaining why one
might want to keep inputs out of the corpus.

Convert From<Corpus> to i32 to a pub fn to_libfuzzer_code() that is
impl on Corpus to avoid accidental conversion.

Fix doc tests.