Refactor: Let MCDC::State have DecisionByStmt and BranchByStmt

- Prune `RegionMCDCBitmapMap` and `RegionCondIDMap`. They are handled
  by `MCDCState`.
- Rename `s/BitmapMap/DecisionByStmt/`. It can handle Decision stuff.
- Rename `s/CondIDMap/BranchByStmt/`. It can be handle Branch stuff.
- `MCDCRecordProcessor`: Use `DecisionParams.BitmapIdx` directly.