fuzz: add initial fuzzers based on libfuzzer

[ghedo]

Supersedes #179. Even Mayhem seems to like this more so it seems like the way to go.

There are two downsides though:

1. cargo-fuzz only seems to work with Rust nightly.
2. cargo-fuzz doesn't support an "initialization" step (e.g. to create quiche Config object), so we need to hack one in using lazy_static.

[ghedo]

Also, the lazy_static hack could be removed if https://github.com/rust-fuzz/libfuzzer-sys/pull/33 (or similar) was merged into libfuzzer-sys.

[camshaft]

You should consider using bolero 😄. I ran into similar downsides and wanted something better. You can see a feature comparison here: https://github.com/camshaft/bolero/#prior-work