Restructure typechecking modules

Some of the kind-checking and type-checking modules have become unruly.

Simon and I propose the new structure:

• KcTyClsDecls will handle kind-checking and generalization of datatype declarations. (That is, kcTyClGroup and children, including related functions in TcHsType)

• TcInstBinds will handle tcInstDecls2 and children

• TcTyConValidity will handle checkValidTyCon and children

• Fold the rest of TcInstDecls (essentially, tcInstDecls1) into what's left of TcTyClsDecls. TcInstDecls is removed.

Trac metadata

Trac field	Value
Version	8.6.2
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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