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	nullAddr# fix for the HEAD
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	*** DO NOT MERGE ***

nullAddr# is simply a name for (Lit nullAddrLit).  Up
to now it has been a PrimOp with the rather stange type
	nullAddr# :: Int# -> Addr#
which discards its argument.  (I think the problem with
nullary primops is to do with the top-level bindings in
PrelPrimOpWrappers.)   And there was a RULE in PrelRules
to rewrite
	nullAddr _ ==> nullAddrLit

It's excessive to make it a PrimOp.  We can just treat it
like unsafeCoerce#, which is made in MkId.lhs.   So I've
done that, and given it the more sensible type
	nullAddr# :: Addr#

I fixed all the occurrences I could find.