Fix egregious error in eta-reduction of data families
This terrible and long-standing bug was shown up by Trac #11148. We are trying to eta-reduce a data family instance, so that we can then derive Functor or Generic. But we were assuming, for absolutely not reason whatsoever, that the type variables were lined up in a convenient order. The fact that it ever worked was a fluke. This patch fixes it properly. Main change is in eta_reduce in TcInstDcls.tcDataFamInstDecl
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