Update theory rewriter ownership, add stats to strings (#4202)

Update theory rewriter ownership, add stats to strings (#4202)

This commit adds statistics for string rewrites. This is work towards proof
support in the string solver. At a high level, this commit adds a pointer to a
`SequenceStatistics` in the rewriters and modifies
`SequencesRewriter::returnRewrite()` to count the rewrites done. In practice,
to make this work requires a couple of changes, some of them temporary:

- We can't have a single `Rewriter` instance shared between different
  `SmtEngine` instances anymore. Thus the `Rewriter` is now owned by the
  `SmtEngine` and calling the rewriter retrieves the rewriter associated with
  the current `SmtEngine`. This is a temporary workaround before we get rid of
  singletons.
- Methods in the `SequencesRewriter` and the `StringsRewriter` are made
  non-`static` because they need access to the statistics instance.
- `StringsEntail` now has non-`static` methods because it needs a reference to
  the sequences rewriter that it can call.
- The interaction between the `StringsRewriter` and the `SequencesRewriter`
  changed: the `StringsRewriter` is now a proper `TheoryRewriter` that inherits
  from `SequencesRewriter` and calls its `postRewrite()` before applying its
  own rewrites (this is essentially a reversal of roles from before: the
  `SequencesRewriter` used to call `static` methods in the `StringsRewriter`).
- The theory rewriters are now owned by the individual theories. This design
  mirrors the `EqualityEngine`s owned by the individual theories.