support for deriving Vector/MVector instances

Since ghc-7.8, the following is no longer possible:

-- simplified example taken from the vector package
class MVectorClass (v :: * -> * -> *) a where
    basicLength :: v s a -> Int

data family MVector s a

data instance MVector s Int -- implementation not important

newtype Age = Age Int deriving (MVectorClass MVector) -- rejected

Following from discussion in #8177, to enable this ghc would need to support data families with representational matching, such that MVector s Int and MVector s Age are representationally equal.

This has broken some code that previously worked, however as there are some workarounds I'm not sure how important it is.

Trac metadata

Trac field	Value
Version	7.8.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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