Add simple parser fuzzer

[killercup]

No description provided.

[killercup]

Protip: If you don't have much time to contribute to a project, have a computer fuzzer do it for you 😉

Edit: Had some time, fixed fuzzer panics

Edit 2: Couldn't find any new stuff after 10M runs with a coverage of 2512 code paths.

[LeopoldArkham]

Thanks Pascal, good to see you here!

Since I have you, any outstanding API requirements you can think of? If so, feel free to mention them in #8

[markcol]

@killercup could you add documentation about how to run the fuzzing tests, or include it as part of the tests, probably ignored by default but included with a filter.

[killercup]

@markcol this uses cargo-fuzz (cargo install cargo-fuzz) and its typical usage (cargo fuzz run parse --release). I can add a short note to the Readme.

Running the fuzzer on CI might be a nice idea, as it's a very fast fuzzer (32k iterations/s), e.g. with a timeout (cargo fuzz run parse --release -- -max_len=4100 -max_total_time=10). @LeopoldArkham, is that something you want? Maybe as another job on Travis?

[LeopoldArkham]

Yes, via Travis seems like the way to do it; Although I'm not certain it should fail the build... Fuzzer errors are kind of esoteric.
I could be swayed either way.