Liberalising IncoherentInstances

[Aleksey Khudyakov]

> Incidentally, I think it'd be an improvement to localise the Overlapping/Incoherent flags to particular instance declarations, via pragmas, something like
>         instance C [a] where
>           {-# ALLOW_OVERLAP #-}
>           op x = ....
>
> Similarly {-# ALLOW_INCOHERENT #-}.   Having -XOverlappingInstances for the whole module is a bit crude., and might be missed when looking at an instance.   How valuable would this be?
>
+1 It would be nice to have same option for undecidable instances as well.