Add roles declarations to allow safe coercions

[kl0tl]

This allows terms of type STArray r a to be coerced to type STArray 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 arrays 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.

[JordanMartinez]

This was merged in #181