The reasonning is that when generating tests, we want  tests for when
the elements do not matter too much.

For instance, if we want tests for choose (x => x == a), and we only
have the test a = 0, then a+a will pass the test, 42 is less likely to
cause these.

- Refactor CEGIS options for clearer defaults

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, ..)