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.

2 files changed, 4 insertions, 6 deletions

diff --git a/src/theory/sep/theory_sep.cpp b/src/theory/sep/theory_sep.cpp
index 1d0ad904c..d90be94f9 100644
--- a/src/theory/sep/theory_sep.cpp
+++ b/src/theory/sep/theory_sep.cpp
@@ -65,11 +65,6 @@ TheorySep::~TheorySep() {
   }
 }
 
-std::unique_ptr<TheoryRewriter> TheorySep::mkTheoryRewriter()
-{
-  return std::unique_ptr<TheoryRewriter>(new TheorySepRewriter());
-}
-
 void TheorySep::setMasterEqualityEngine(eq::EqualityEngine* eq) {
   d_equalityEngine.setMasterEqualityEngine(eq);
 }
diff --git a/src/theory/sep/theory_sep.h b/src/theory/sep/theory_sep.h
index df3294882..ce1498f52 100644
--- a/src/theory/sep/theory_sep.h
+++ b/src/theory/sep/theory_sep.h
@@ -23,6 +23,7 @@
 #include "context/cdhashset.h"
 #include "context/cdlist.h"
 #include "context/cdqueue.h"
+#include "theory/sep/theory_sep_rewriter.h"
 #include "theory/theory.h"
 #include "theory/uf/equality_engine.h"
 #include "util/statistics_registry.h"
@@ -56,6 +57,8 @@ class TheorySep : public Theory {
   //whether bounds have been initialized
   bool d_bounds_init;
 
+  TheorySepRewriter d_rewriter;
+
   Node mkAnd( std::vector< TNode >& assumptions );
 
   int processAssertion( Node n, std::map< int, std::map< Node, int > >& visited, 
@@ -66,7 +69,7 @@ class TheorySep : public Theory {
   TheorySep(context::Context* c, context::UserContext* u, OutputChannel& out, Valuation valuation, const LogicInfo& logicInfo);
   ~TheorySep();
 
-  std::unique_ptr<TheoryRewriter> mkTheoryRewriter() override;
+  TheoryRewriter* getTheoryRewriter() override { return &d_rewriter; }
 
   void setMasterEqualityEngine(eq::EqualityEngine* eq) override;