Data flow: Refactoring + performance improvements

- Introduce `ReadTaintNode` and `TaintStoreNode` to simplify logic for taint
  getters and taint setters, respectively.
- `nodeCandFwd2`: Restrict `stored` column after a read, based on what it might
  be before a store of the same field.
- `nodeCand2`: Restrict `read` column (renamed from `stored`) after a store, based
  on what it might be after a read of the same field.
- Move big step predicates into a `LocalFlowBigStep` module.
- Define predicates by dispatch in `AccessPath[Front]` class.
- `flowCandFwd0`: Restrict `apf` column after a read, as it should be able to match
  a Boolean `read` column from `nodeCand2`.
- `flowFwd0`: Restrict columns `ap` and `apf` after a read, by introducing a
  `flowConsCandFwd` predicate (similar to what is done in the previous pruning steps).
- `flowFwd0`: Restrict columns `ap` and `apf` after a store, by introducing a
  `flowConsCand` predicate (similar to what is done in the previous pruning steps).

Author: hvitved