Implement a custom mutator that takes a list of fixed-size chunks which
are mutated with a given probability. This makes sure that values like
parser options or failure position are mutated regularly even as the
fuzz data grows large. Values can also be adjusted temporarily to make
the fuzzer focus on failure injection, for example.
Thanks to David Kilzer for the idea.
Remove the libfuzzer max_len option which doesn't apply to other
fuzzing engines. Enforce the maximum length directly in the fuzz
targets. For the xml target, lower the maximum when expanding entities
to avoid timeout and OOM errors.
