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/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.