This patch makes it possible for core-to-core passes to emit
proper error messages and warnings.

  * New function CoreMonad.warnMsg

  * CoreMonad.warnMsg and errorMsg now print a proper warning/error
    message heading.

  * CoreMonad carries a SrcSpan, which is used in warning/error
    messages.  It is initialised to be the source file name, but
    a core-to-core pass could set it more specifically if it had
    better location information.

There was a bit of plumbing needed to get the filename to the
right place.