Add debuginfo representation of classes

[jdm]

It's worth waiting until classes are in a complete state, but it'll need to be done at some point.

[catamorphism]

Classes are represented exactly like records at runtime, right now, but once I finish making classes-implementing-ifaces work, classes will optionally have vtables. (Just so you know...)

[sanxiyn]

I don't think Rust has classes... Can someone rewrite appropriately?

[jdm]

We'll open a new one if impls or traits need particular handling.