Caching

[algebraic-dev]

The rust regex library uses caching for improving a little bit the performance. I think that we could add something for it.

[pandaman64]

For those who are interested in shaving off unnecessary allocations:

• NFA.captureNext discards the sparse sets and saveSlots every time it's called, which can be reused across the calls
• Also, I think it's initSave reallocates an empty array due to the dependency on saveSize. We should allocate it once for one captureNext call (ideally once for each size).
• (Difficult) I believe Option Pos incurs a heap allocation. Since the string length is bounded in practice, we can embed .none into Pos and just use usize under the food.

I'd recommend #11 first though so that we have a benchmark to measure the effect of the optimizations.