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.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/smt/smt_engine.cpp b/src/smt/smt_engine.cpp
index 2e1716543..03d9409be 100644
--- a/src/smt/smt_engine.cpp
+++ b/src/smt/smt_engine.cpp
@@ -802,6 +802,7 @@ SmtEngine::SmtEngine(ExprManager* em)
       d_theoryEngine(nullptr),
       d_propEngine(nullptr),
       d_proofManager(nullptr),
+      d_rewriter(new theory::Rewriter()),
       d_definedFunctions(nullptr),
       d_fmfRecFunctionsDefined(nullptr),
       d_assertionList(nullptr),