Add round-trip fuzzers

[davidben]

It should be the case that, for all byte strings b, ascii2der(der2ascii(b)) = b. Write a fuzzer for this. Also write a fuzzer to check that ascii2der doesn't panic.

One nuisance this'll run into is that I'm sure both functions stack overflow or go quadratic when given the wrong inputs. For the project's use cases, I hadn't considered this important, but the fuzzers will likely get stuck in that rabbithole.