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	Fix another bug in the squash-newtypes story.
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[This one was spotted by Marcin, and is now enshrined in test tc130.]

The desugarer straddles the boundary between the type checker and
Core, so it sometimes needs to look through newtypes/implicit parameters
and sometimes not.  This is really a bit painful, but I can't think of
a better way to do it.

The only simple way to fix things was to pass a bit more type
information in the HsExpr type, from the type checker to the desugarer.
That led to the non-local changes you can see.

On the way I fixed one other thing.  In various HsSyn constructors
there is a Type that is bogus (bottom) before the type checker, and
filled in with a real type by the type checker.  In one place it was
a (Maybe Type) which was Nothing before, and (Just ty) afterwards.
I've defined a type synonym HsTypes.PostTcType for this, and a named
bottom value HsTypes.placeHolderType to use when you want the bottom
value.