remove-unused

[Stephen Weeks]

> 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).

Makes sense.  There's still some room for speedups by avoiding
repeated lookups of the same property, but I'll save that for another
day.