[m-dev.] diff: reorder items in the NEWS file

[Zoltan Somogyi]

> NEWS:
> 	Undo Zoltan's (accidental?) changes to the ordering.
> 	Changes to language features are now documented in the same
> 	order as those features are documented in the
> 	Mercury language reference manual.

Actually, the changes were not accidental. I moved the more important stuff
closer to the front. I think this matters much more than any relationship
to the order of things in the manual.

Zoltan.