Implement "Meaningful main return types" proposal

The GHC Steering Committee has accepted the proposal "Meaningful main return types", which introduces:

• a new extension MeaningfulMainReturn under which main is required to have a type IO (), IO Void or IO ExitCode, with the latter arranging to return the ExitCode on program termination; and
• a new on-by-default warning -Wambiguous-main-return which will complain if main has a type other than IO () or IO Void (and MeaningfulMainReturn is not in effect).