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        Implement recursive do-notation
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This commit adds recursive do-notation, which Hugs has had for some time.

	mdo { x <- foo y ;
	      y <- baz x ;
	      return (y,x) }

turns into

	do { (x,y) <- mfix (\~(x,y) -> do { x <- foo y;
					    y <- baz x }) ;
	     return (y,x) }

This is all based on work by Levent Erkok and John Lanuchbury.

The really tricky bit is in the renamer (RnExpr.rnMDoStmts) where
we break things up into minimal segments.  The rest is easy, including
the type checker.

Levent laid the groundwork, and Simon finished it off. Needless to say,
I couldn't resist tidying up other stuff, so there's no guaranteed I
have not broken something.