Add fuzz testing

[maxammann]

I was wondering to which extend this library is fuzz tested internal or externally. I could not find any references to continuous fuzzing being setup. Did you consider integrating into OSS-fuzz?

EDIT: Found this reference:https://github.com/facebookincubator/fizz/commit/a9d0289c9c91170a2bea70e67899935045f28f21