Another big commit from Simon.  Actually, the last one
didn't all go into the main trunk; because of a CVS glitch it
ended up in the wrong branch.

So this commit includes:

* Scoped type variables
* Warnings for unused variables should work now (they didn't before)
* Simplifier improvements:
	- Much better treatment of strict arguments
	- Better treatment of bottoming Ids
	- No need for w/w split for fns that are merely strict
	- Fewer iterations needed, I hope
* Less gratuitous renaming in interface files and abs C
* OccName is a separate module, and is an abstract data type

I think the whole Prelude and Exts libraries compile correctly.
Something isn't quite right about typechecking existentials though.