Add roles declarations to allow safe coercions

[kl0tl]

This allows terms of type ST r a and STRef r a to be coerced to type ST r b and STRef r b when Coercible a b holds, hence allowing the zero cost coerce instead of map wrap and map unwrap to introduce and eliminate newtypes under ST computations and mutable references for instance.

I couldn’t think of a case where a phantom roled region is an issue but in doubt I preferred to keep it nominal.

[hdgarrood]

Included in #37