Add LibAFL

[louismerlin]

In this PR I'll add a third fuzzer to our roster: LibAFL.

I'll start by making it very simple and include corpus sharing and basic monitoring.

In the future, we also want users to bring their own LibAFL-based fuzzer to add to the roster; technical details are still a bit fuzzy but I think we can get there. This will be done in another PR.

• [x] LibAFL fuzzer that gets launched alongside the others
• [x] Basic monitoring
• [x] Multi-threaded
• [x] Corpus sharing
• [x] Dictionary
• [ ] Input size limit
• [ ] Timeout
• [x] Coverage monitoring
• [ ] Fix coverage (MAX_EDGES_NUM is too high)
• [x] Fix flickering (push to String then print)
• [ ] Add --libafl, --afl, --honggfuzz flags

Partially addresses #34.