remove-unused

[Matthew Fluet]

> I must admit to still being confused.  I have checked in my changes to
> remove-unused as part of getting checkHandlers up and running (it now
> passes all tests).  Could you make the changes you think are still
> needed and I will take a look to see if the code makes sense to me?

I checked in the fixes.  There are some more detailed comments in the
commit log.  The only meaningful changes are in the rewriting of tail
calls when the caller and callee don't have the same return types (or,
more precisely, they won't have the same return types when we filter out
the unused returns).