- Partial support for CVC4

- Both Expr-based solvers and z3-based solvers rely on same template
  generation routines and unrolling strategy. Templates and unrolling
  bank is refactored in solvers.templates._

- Encoding is done through TemplateEncoder. Performance highly relies on
  an efficient TemplateEncoder.substitute() implementation.

- Implement smt and smt-z3/smt-cvc4 as options for --solvers

- Uses a main solver sources for verification/CLP, taking into account
  --solvers

- Implement Z3-SMT sets

- Add tests for Z3-SMT

More precise tracing, especially for in-IDE exploration of execution

Previously if the choose was in the following context:

```
a match {
    case Cons(h, t) =>
    case _ =>
        choose()
}
```

its path condition would include:

```
!(a.isInstanceOf[Cons] && a.head == h && a.tail == t)
```

which causes mostly unconstrained variables `h` and `t` to show up in the
synthesis problem.