Add sortOn/coerce rule

When the argument to sortOn is very cheap, we're better off not decorating the list. Detecting this is not easy in general, but it's trivial for coercions. We should probably offer something like

{-# RULES
"sortOn/coerce" forall b xs.
    sortOn @_ @b coerce xs = sortWith (coerce @_ @b) xs
 #-}

As far as I can tell, we can't use type application directly like that, but we can go via Proxy#.

PrelRules could do better, of course, catching selector functions, but I don't think we want to go that far.

Trac metadata

Trac field	Value
Version	8.6.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Core Libraries
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