Allow oneShot to work with unboxed types

Currently oneShot requires that both the argument type and the return type of the given function have the kind *. It would be nice if I could use unlifted types there.

The following program demonstrates this:

{-# LANGUAGE MagicHash #-}
module Foo where

import GHC.Exts
import GHC.Magic

f0 :: Int -> Int
f0 = oneShot $ \n -> n -- OK

f1 :: Int# -> Int
f1 = oneShot $ \n# -> I# n# -- Error, the argument type is unlifted

f2 :: Int -> Int#
f2 = oneShot $ \(I# n#) -> n# -- Error, the result type is unlifted

Trac metadata

Trac field	Value
Version	7.10.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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