Classes: implement destructors

[catamorphism]

Implement dtors for classes, then get rid of resources (rewriting resource-using code to use classes with dtors instead).

[catamorphism]

Status: destructors are implemented, but can't be called correctly from the cycle collector (resource-cycle fails when ported to use classes). I think this is probably easy to fix for someone who understands the RTS. Not all test cases have been ported yet. I'm deferring this for the moment to work on more urgent bugs. If someone else worked on it, I wouldn't object.

[catamorphism]

I'll just consider this done except for the cycle collector part, and file more specific bugs.