Add module for use with rebindable syntax

[pseudonom]

This is pretty convenient for use with RebindableSyntax.

[endgame]

I came here to leave almost exactly the same PR. Is there any chance it can get merged?

Also, there seems to be some weird interactions with return and -XRebindableSyntax. This module fails to compile:

{-# LANGUAGE RebindableSyntax #-}

module Lib where

import Prelude (IO)

foo :: IO ()
foo = do
  pure ()

This is the error (GHC 8.0.2):

error:
    Not in scope: ‘return’
    Perhaps you want to add ‘return’ to the import list
    in the import of ‘Prelude’ (src/Lib.hs:5:1-19).