Add StabilityCheckStage

[tokatoka]

This PR adds a StabilityCheckStage which does these two things

• Measure stability of the current scheduled testcases
• If there's in-stability discovered, we set the history map of that entry to MAX, so that later fuzzing would not be influenced by this flaky map entry.

Basically this is the ported code from https://github.com/AFLplusplus/LibAFL/blob/main/crates/libafl/src/stages/calibrate.rs (or the equivalent code from AFL++), but made it more simple and readable for fuzzamoto to use.

I adjusted the monitor so you can see the result like this when you run it

📊 time: 7m-35s (x1) execs: 49782 cov: 3.936% corpus: 685 exec/sec: 111.7 bugs: 0 stability: 99.350% 

[dergoegge]

What are the changes you made to the stage, compared to the one in libafl? Is there a reason why we can't use the stage from the library?

[tokatoka]

Basically, the biggest difference is that I removed some parts that is unnecessary for fuzzamoto, especially the parts for scheduler's metadata (because I think we can't just reuse libafl's scheduler and apply it to fuzzamoto)

The reason I did not want to plug the module of libafl into fuzzamoto is that reimplementing (and choosing whatever parts we need) makes it easier to extend.
For example, in one example use-case that I pushed in #89, I extended this StabilityCheckStage to dump the filenames of unstable test cases to examine them later, but achieving this would be not as straightforward if we just plugged the libafl's CalibrationStage