Emit warnings about unused do-bindings

It would be nice if unused do-bindings (i.e. non-unit values) such as getLine in

unused :: IO ()
unused = do
  getLine
  return ()

would emit a warning similar to GHC (with -Wall):

UnusedDo.hs:5:3: warning: [-Wunused-do-bind]
    A do-notation statement discarded a result of type ‘String’
    Suppress this warning by saying ‘_ <- getLine’
  |
5 |   getLine
  |   ^^^^^^^

This would also be a good candidate for a quick fix as proposed in #92.

Related: #62.