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

Notice the removal of type parameter in TemplateGenerator.groupWhile.
I am not sure what was intended (this method is currently unused).

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.

Leon now matches Scala semantics of Int as 32 bits bit-vectors.
This commits modifies the semantics of IntLiteral to be treated
as 32 bits integer everywhere (solver, evaluator, ...).

Introduces a new literal type, InfiniteIntegerLiteral, representing
a natural integer. The front-end maps the use of BigInt to these
new trees, and the solver properly handles them as mathematical
integers.

The behaviour of many regression tests changes due to this new
semantics. In particular many of them now timeout because they
are no longer proving properties over mathematical integers. This
commit updates the tests to reflect this new semantics.