Also, call unveilUntyped consistently when constructing Trees.

Use constructors/extractors everywhere (except solvers).
Eliminate any Tuples with <2 elements from Leon.
Deprecate rewriteTuples.
Rewrite FiniteLambda. Not necessarily better, just clearer.

Function invocation not attached to any value in
a statement block should be kept for potential
side effects. The committed regression test
illustrates the issue.

Additionally, become consistent in the name XLang
in various files of Leon.

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.