Add fuzzing

[jneem]

Running the fuzzer requires cargo fuzz installed. It's incompatible with lto, so you'll have to comment out the lto = true line in Cargo.toml. Then running cargo fuzz run parse should run the fuzzer.

[jplatte]

Rather than editing the Cargo.toml, setting CARGO_PROFILE_RELEASE_LTO=false should also work, right? Could document that somewhere as part of the PR if it works.

[Ethiraric]

Awesome, thanks! If I had known it was that simple, I'd have added that a while back.

I'll add a justfile command that sets the flags right.