add WARNING pragma

[ijones]

It might be nice to have a WARNING pragma in place of or in addition to the DEPRECATED pragma that just allows you to issue a generic warning:

{-# WARNING f, C, T "These are deprecated" #-}
{-# WARNING g "This has terrible performance" #-}
{-# WARNING T "We had to expose this type for bad reasons, don't use it." #-}
{-# WARNING preConf "This is an unstable interface. May become deprecated." #-}
{-# WARNING fromJust "Calls the error function.  use maybe for safety" #-}
module Wibble {-# WARNING "Use Wibble_1 instead" #-}

This is more general than the DEPRECATED pragma.

[simonpj]

This could be easily done if we can agree about what we want.

Should "WARNING" be "WARN_USE" or "WARN_USE_OF" or "WARN_AT_USE" or something like that? Something more self explanatory.

(Or use hyphens? "WARN-USE" or "WARN-USE-OF" etc.)

Then DEPRECATED becomes "WARN-USE" with an automatic prefix "x is deprecated.". Right?

[igloo]

WARNING pragma added