perf(coverage): cache current HitMap, reserve when merging

[DaniPopes]

Avoid one of the two hashmap lookups in CoverageCollector::step() by caching the current contract's hashmap in a separate field. This unfortunately has to use a raw pointer and unsafe because self-referential structs are not possible.

Additionally, add some reserve calls and initialize with some initial capacity to reduce the impact of reserve(1) in entry calls.

10-20% faster forge coverage on solmate.

[mattsse]

cool, makes sense,
some doc suggestions

[DaniPopes]

Addressed in #9474