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/sat/sat_solver/inc_sat_solver.cpp

@@ -463,6 +463,10 @@ public:
         }
         return fmls;
     }
+
+    expr* congruence_next(expr* e) override { return e; }
+    expr* congruence_root(expr* e) override { return e; }
+
     
     lbool get_consequences_core(expr_ref_vector const& assumptions, expr_ref_vector const& vars, expr_ref_vector& conseq) override {
         init_preprocess();

src/sat/sat_solver/sat_smt_solver.cpp

@@ -476,6 +476,9 @@ public:
             set_reason_unknown(m_solver.get_reason_unknown());
         return fmls;
     }
+
+    expr* congruence_next(expr* e) override { return e; }
+    expr* congruence_root(expr* e) override { return e; }
     
 
     lbool find_mutexes(expr_ref_vector const& vars, vector<expr_ref_vector>& mutexes) override {