Add support for AFL Persistent Mode

[domenukk]

Great Project!
One problem I have is that startup for my binary is pretty slow.
The way I understood it, afl is given the actual binary to execute and will fork on _start.
Since the afl.rs api already wraps every call in a closure, it would map perfectly to Persistent Mode:
https://github.com/mirrorer/afl/blob/master/llvm_mode/README.llvm#L139
That way execution speed (at least the setup) should be a lot quicker.
A good middle ground might be the start the delayed fork server at the rust afl entrypoint.

[PaulGrandperrin]

Maybe you should try cargo-fuzz or honggfuzz-rs.

They both use persistent mode.

Disclaimer: I'm the author of honggfuzz-rs.

[PaulGrandperrin]

Also I want to point out that I'm working on making this happen in AFL.rs.
I have a working version but the speedup is not that great...
For instance fuzzing url on my computer, using 1 thread:
AFL.rs w/o persistent mode: ~2000/s
AFL.rs w/ persistent mode: ~10000/s
Honggfuzz w/ persistent mode: ~ 60000/s

[frewsxcv]

Done in #137