FiniteSet.contains: use Eq to check inclusion

If an object defines the _eval_Eq method, it can be used
to test whether an object is not in the set.
Interval: relax test for inclusion; do symbolic reversal test
use prec with _eval_evalf, not evalf (which uses dps)
Set.as_relational: slightly simplify code
Sets: shorten line lengths
Sets: add tests
quick