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/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 {