Equality constraints for types

is_type gives the trait Is<X> which acts like an equality constraint.

There's also functions to move between these two equivalent types, as Rust will still not realise these types are

The constraint X : Is<Y> requires that X == Y.

The idea was stolen from this comment on equality constraints in where clauses which contains an example use case.

Note that into_val and from_val are basically into and from, but for this trait to work universally we need a universal instance, which one can't define on Into or From.