First steps towards DEPRECATED before Rosebank (12yrs) takes its toll.
Nothing very functional yet, but at least hsc can be compiled and it
still compiles the Prelude.

Parsing the pragma turned out to be a little bit more complicated than
expected, here the comment from Parser.y:

   The place for module deprecation is really too restrictive, but if it
   was allowed at its natural place just before 'module', we get an ugly
   s/r conflict with the second alternative. Another solution would be the
   introduction of a new pragma DEPRECATED_MODULE, but this is not very nice,
   either, and DEPRECATED is only expected to be used by people who really
   know what they are doing. :-)

Net result: Module deprecation is allowed exactly behind the module's
name and nowhere else. I probably have to think a little bit more
about this some day...