Optimize coercionKind

Currently, the ForAllCo case of coercionKind does an inefficient one-variable substitution. This could be improved by looking for nested ForAllCos.

Furthermore, perhaps we don't need an in-scope set and the full substitution machinery here. The subst is simply propagating the update of a tyvar's kind. No structural changes at all. No need for smart coercion constructors or other processing. So if we're going to optimize this, it might be worth making a specialized version of subst_ty and subst_co that operate over a VarEnv Var instead of a full substitution.

Trac metadata

Trac field	Value
Version	7.10.3
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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