experimental feature to access congruence closure of SimpleSolver

This update includes an experimental feature to access a congruence closure data-structure after search.
It comes with several caveats as pre-processing is free to eliminate terms. It is therefore necessary to use a solver that does not eliminate the terms you want to track for congruence of. This is partially addressed by using SimpleSolver or incremental mode solving.

```python
from z3 import *
s = SimpleSolver()
x, y, z = Ints('x y z')
s.add(x == y)
s.add(y == z)
s.check()
print(s.root(x), s.root(y), s.root(z))
print(s.next(x), s.next(y), s.next(z))
```

src/solver/combined_solver.cpp

@@ -275,6 +275,10 @@ public:
         return m_solver2->cube(vars, backtrack_level);
     }
 
+    expr* congruence_next(expr* e) override { switch_inc_mode(); return m_solver2->congruence_next(e); }
+    expr* congruence_root(expr* e) override { switch_inc_mode(); return m_solver2->congruence_root(e); }
+
+
     expr * get_assumption(unsigned idx) const override {
         unsigned c1 = m_solver1->get_num_assumptions();
         if (idx < c1) return m_solver1->get_assumption(idx);

src/solver/solver.h

@@ -238,6 +238,15 @@ public:
 
     virtual expr_ref_vector cube(expr_ref_vector& vars, unsigned backtrack_level) = 0;
 
+    /**
+       \brief retrieve congruence closure root.
+    */
+    virtual expr* congruence_root(expr* e) = 0;
+
+    /**
+       \brief retrieve congruence closure sibling
+    */
+    virtual expr* congruence_next(expr* e) = 0;
 
     /**
        \brief Display the content of this solver.

src/solver/solver_pool.cpp

@@ -262,6 +262,9 @@ public:
 
     expr_ref_vector cube(expr_ref_vector& vars, unsigned ) override { return expr_ref_vector(m); }
 
+    expr* congruence_next(expr* e) override { return e; }
+    expr* congruence_root(expr* e) override { return e; }
+
     ast_manager& get_manager() const override { return m_base->get_manager(); }
 
     void refresh(solver* new_base) {

src/solver/tactic2solver.cpp

@@ -136,6 +136,9 @@ public:
         return expr_ref_vector(get_manager());
     }
 
+    expr* congruence_next(expr* e) override { return e; }
+    expr* congruence_root(expr* e) override { return e; }
+
     model_converter_ref get_model_converter() const override { return m_mc; }
 
     void get_levels(ptr_vector<expr> const& vars, unsigned_vector& depth) override {