Refactor PerfConfig

[arthurpaulino]

Our PerfConfig can better represent the parallelizations that we have:

• Parallel witness generation for the slots
• Parallel witness generation for the frames within a MultiFrame
• Folding in parallel with witness generation

Therefore, our PerfConfig would better capture what actually happens with more meaningful attributes. For example:

struct PerfConfig {
  slots: Flow,
  frames: Flow,
  witgen_vs_folding: Flow,
}

Then, our canned configs would be:

• FullySequential
• ParallelSlots
• ParallelFrames
• ParallelWitGenVsFolding
• FullyParallel

[samuelburnham]

Sounds great, please ping me for review on the eventual PR so I can check it integrates with the user config and env vars