eq method on tuples

[dullbananas]

(a1, b1).eq((a2, b2)) would be much cleaner than a1.eq(a2).and(b1.eq(b2)).

More importantly, this would also allow using the type dsl::Eq<(a1, b1), (a2, b2)> instead of dsl::And<dsl::Eq<a1, a2>, dsl::Eq<b1, b2>>.

[weiznich]

Duplicate of #3723