Add infrastructure to automatically run fuzzers in CI,
and implement a simple fuzzing test based on triggering all (most)
public APIs in a randimized way.
As far as I was able to try it catches the previous unsoundness issues
in a matter of seconds. This can be tried by changing the `path = "../"` dependency to
`version = "=0.6.3"` etc. and running the fuzzer manually. (Note: You'll need
to tweak the `Cargo.lock` to allow downloading the yanked versions).
Related to #124