****	MERGE WITH 5.00 BRANCH     ********

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	Make parallel list comprehensions work
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There were two bugs

1.  The desugaring in DsListComp was generating code that failed Lint.
    I've restructured it quite a lot.

2.  More seriously, in a ParStmt, the last 'stmt' may be a guard;
    but previously both guards and the result of a list comprehension
    were encoded as an ExprStmt (see HsExpr.Stmt), using the fact that
    the stmt was last in the list to make the difference between a guard
    and a result.  But in parallel list comp this isn't right:

	[ e | x <- xs, guard | y <- ys ]

    Here 'guard' is last in its list, but isn't an overall result.

    The sensible fix is to properly distinguish
	"here's the answer" 			 (ResultStmt)
	"here's a guard or an imperative action" (ExprStmt)

    The fix is rather easy, but touched quite a lot of files.  On the
    way I tidied up the parser a little.