Implemented python custom mutators for libfuzzer mode (a potential fix for https://github.com/dvyukov/go-fuzz/issues/319)

[personnumber3377]

Hi!

I did a quick blog post where I implemented python custom mutators for go-fuzz libfuzzer mode: https://personnumber3377.github.io/projects/implementing_python_mutators_for_go_fuzz.html Please read the instructions carefully. I had to do some janky stuff which could probably be implemented in a cleaner way to make it work. I ignored multiple definitions for example.

My fork is here: https://github.com/personnumber3377/go-fuzz/tree/master feel free to modify it however you like before merging to this main repo.