Improve enumFromX support for OverloadedLists

At present, OverloadedLists desugars [m..n] as fromList (enumFromTo m n). This is sometimes okay, but terrible for Data.Sequence. In particular, I want, for example,

[m..n] :: Seq Int

to end up as fromFunction (n-m+1) (+m), which performs much better. There are a few approaches that look reasonable:

1. Try to catch fromList (enumFromTo m n) before enumFromTo becomes eftInt. This does not, unfortunately, seem to work.
2. Grab eftInt after it gets written back from eftIntFB. This would probably work, but it's currently impossible because eftInt is not exported from GHC.Exts or even GHC.Enum.
3. Add enumFromTo, etc., to the IsList class. Default to the current behavior, but if possible use fromListN for well-known Enum types.

Trac metadata

Trac field	Value
Version	7.9
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Core Libraries
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CC	core-libraries-committee@haskell.org
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