improve hash reasoning and performance

[Daejun Park]

1. evaluate symbolic terms with aggressive constant propagation, when concrete values are expected
   e.g., if we have x == 1 in the path condition, evaluate x + 1 to 2, for memory access, resolving external call target, etc.

2. for constant hash expressions, add equalities between the term and its evaluation
   e.g., for sha3(1), add sha3(1) == <hash-image> in the constraints

3. lazily generate the no hash collision axioms only when hash terms appear in branching conditions

4. separate branching conditions from internally generated constraints