Right now we expect crashes on invalid parses, we just don't try to
handle it in general even though the long-term intent is we should
handle semantics for bad parses. However, in theory, we should be
correctly handling code that parses as valid and that's probably more
interesting to fix bugs for. So this starts trying to fuzz that space of
valid parses.
Fuzzer corpus is based on explorer tests, with one merge run.
