Eliminate MutableTyped from Expr's. Expr.getType is now a val.
Variables don't have a mutable type.
Separate representation of empty and nonempty Sets, Maps, Multisets, and Arrays.
Introduce more generic constructors/ extractors for these types.
Simplify Map builder in NAryOperator.
Deprecate some deprecated Expr's.
Represent String literals as Lists.
Make some tests consistent with typing limitations in Leon.

- evaluate it to simplest value
- solve it to arbitrary value
- Use within CEGLESS as bank of exprs
- Avoid GuidedCloser if non-det expr (contains choose, holes, ..)

+ various fixes for repair

 - Some engineering fixes here and there
 - Make sure we remove witnesses before sending out to solver
 - Speed-up CEGIS:
   - Order tests based on likelihood to falsify input
   - Only assert wrongPrograms (C-Ex) when necessary