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, 5 insertions, 6 deletions

diff --git a/src/theory/bv/theory_bv.cpp b/src/theory/bv/theory_bv.cpp
index 27718b63f..94fc1e34c 100644
--- a/src/theory/bv/theory_bv.cpp
+++ b/src/theory/bv/theory_bv.cpp
@@ -112,11 +112,6 @@ TheoryBV::TheoryBV(context::Context* c,
 
 TheoryBV::~TheoryBV() {}
 
-std::unique_ptr<TheoryRewriter> TheoryBV::mkTheoryRewriter()
-{
-  return std::unique_ptr<TheoryRewriter>(new TheoryBVRewriter());
-}
-
 void TheoryBV::setMasterEqualityEngine(eq::EqualityEngine* eq) {
   if (options::bitblastMode() == options::BitblastMode::EAGER)
   {
diff --git a/src/theory/bv/theory_bv.h b/src/theory/bv/theory_bv.h
index ff1c9245a..bc54a09e7 100644
--- a/src/theory/bv/theory_bv.h
+++ b/src/theory/bv/theory_bv.h
@@ -26,6 +26,7 @@
 #include "context/cdlist.h"
 #include "context/context.h"
 #include "theory/bv/bv_subtheory.h"
+#include "theory/bv/theory_bv_rewriter.h"
 #include "theory/bv/theory_bv_utils.h"
 #include "theory/theory.h"
 #include "util/hash.h"
@@ -72,7 +73,7 @@ public:
 
   ~TheoryBV();
 
-  std::unique_ptr<TheoryRewriter> mkTheoryRewriter() override;
+  TheoryRewriter* getTheoryRewriter() override { return &d_rewriter; }
 
   void setMasterEqualityEngine(eq::EqualityEngine* eq) override;
 
@@ -259,6 +260,9 @@ public:
 
   void checkForLemma(TNode node);
 
+  /** The theory rewriter for this theory. */
+  TheoryBVRewriter d_rewriter;
+
   friend class LazyBitblaster;
   friend class TLazyBitblaster;
   friend class EagerBitblaster;