Allow family instances in an hs-boot file

Compiling an .hs-boot file with a data instance leads to this error:

    Illegal family instance in hs-boot file

Yet, data instances make good sense in an hs-boot file, in case another module wants to access a constructor declared in the instance.

Furthermore, it may sometimes be desirable to allow type family instances in a hs-boot file, as well, in case some including modules need the instance for type simplification.

Trac metadata

Trac field	Value
Version	7.7
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
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