Upgrade for PureScript 0.12

[paluh]

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[paluh]

@Thimoteus I'm not sure if I'm able to preserve instance for identity. I mean this:

instance coercibleId :: Coercible a a where
  coerce = identity

because with this I think we are not able to provide any other instance.

[Thimoteus]

What happens if you put it last in the instance chain? Can you show that it'll never get picked?

[paluh]

This is not really the problem. The problem is that it seems to overlap with any external instance...

[Thimoteus]

Hmm, that's a good point...

[paluh]

I'm migrating purescript-snail currently and I have another problem with overlapping instances for Optional and Maybe:

instance coercibleMaybeOptional :: Coercible (Maybe a) (Optional a) where

To be honest my focus is to migrate purescript-snail and literate-purescript at the moment, so I'm going to use... Newtype there :-P

[Thimoteus]

Yeah, I think those are old enough to have been written before Newtype made it into PS, but I'm not sure.