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.

3 files changed, 9 insertions, 3 deletions

diff --git a/src/theory/sets/theory_sets.cpp b/src/theory/sets/theory_sets.cpp
index 0b9e6d934..f4b265d98 100644
--- a/src/theory/sets/theory_sets.cpp
+++ b/src/theory/sets/theory_sets.cpp
@@ -46,9 +46,9 @@ TheorySets::~TheorySets()
   // Do not move me to the header. See explanation in the constructor.
 }
 
-std::unique_ptr<TheoryRewriter> TheorySets::mkTheoryRewriter()
+TheoryRewriter* TheorySets::getTheoryRewriter()
 {
-  return std::unique_ptr<TheoryRewriter>(new TheorySetsRewriter());
+  return d_internal->getTheoryRewriter();
 }
 
 void TheorySets::finishInit()
diff --git a/src/theory/sets/theory_sets.h b/src/theory/sets/theory_sets.h
index a55a22600..5fc1a61a3 100644
--- a/src/theory/sets/theory_sets.h
+++ b/src/theory/sets/theory_sets.h
@@ -42,7 +42,7 @@ class TheorySets : public Theory
              const LogicInfo& logicInfo);
   ~TheorySets() override;
 
-  std::unique_ptr<TheoryRewriter> mkTheoryRewriter() override;
+  TheoryRewriter* getTheoryRewriter() override;
 
   /** finish initialization */
   void finishInit() override;
diff --git a/src/theory/sets/theory_sets_private.h b/src/theory/sets/theory_sets_private.h
index 5ef8c4825..ab9071793 100644
--- a/src/theory/sets/theory_sets_private.h
+++ b/src/theory/sets/theory_sets_private.h
@@ -26,6 +26,7 @@
 #include "theory/sets/inference_manager.h"
 #include "theory/sets/solver_state.h"
 #include "theory/sets/theory_sets_rels.h"
+#include "theory/sets/theory_sets_rewriter.h"
 #include "theory/theory.h"
 #include "theory/uf/equality_engine.h"
 
@@ -161,6 +162,8 @@ class TheorySetsPrivate {
 
   ~TheorySetsPrivate();
 
+  TheoryRewriter* getTheoryRewriter() { return &d_rewriter; }
+
   void setMasterEqualityEngine(eq::EqualityEngine* eq);
 
   void addSharedTerm(TNode);
@@ -279,6 +282,9 @@ class TheorySetsPrivate {
    * involving cardinality constraints is asserted to this theory.
    */
   bool d_card_enabled;
+
+  /** The theory rewriter for this theory. */
+  TheorySetsRewriter d_rewriter;
 };/* class TheorySetsPrivate */