Make sure classes can be compared

[catamorphism]

This is apropos of a FIXME in test/run-pass/binops.rs that says "uncomment this code when we can compare resources". When resources get replaced with classes, we'll need to be able to compare classes.

[catamorphism]

I have a patch ready (awaiting a snapshot) that implements == and != for classes. For classes with dtors, equality means pointer equality, but for classes without dtors, it means structural equality. I'm not sure whether the comparison operators should be defined on classes.