Allow more than one target score to be tracked

[Seelengrab]

It may be beneficial to track more than one good attempt, which can be attempted to be improved independently from other good-ish attempts. See e.g. IJON: Exploring Deep State Spaces via Fuzzing, which implements a form of coverage based fuzzing through multiple (user annotated) targets, allowing their fuzzer to beat Level 3-4 of the first Super Mario Bros.:

(theirs is the one at the bottom, while the one at the top struggles with the large state space).

This feature would:

• Extend the number of tracked targets from 1 to (just throwing out a number here) 256
  • These would be exposed as a second argument specifying the slot to store the score in; the default would be slot 1.
• Make target optimization smarter by choosing amongst the tracked targets which would be a good case for further expansion, again trying to maximize its score
• Rework how was_better is calculated, since tracking more than one target means the calculation whether any given example actually was better becomes more complicated.
  • Perhaps this can be done by allowing a custom overall scoring function, given the calculated scores?
  • If none is given, this could default to prioritizing the first slot over the second etc. Receiving a custom function for deciding whether the example was better than the last allows users to decide that they want to ignore e.g. slot 33 in almost all cases, except when slot 40 is >500, for example.
  • Allowing a custom function should not be required to consider this feature implemented, and can be done at a later time. This should simplify a first implementation quite a bit, since we can postpone thoughts on how to expose this information to a user.