Support typedefs in the interpreter

[samestep]

This is a blocker for #34. We need to figure out how to deal with the fact that a single Typedef may be referenced by multiple different functions. Ideally we'd find a way to do this that doesn't require HashMap. Or, maybe the best solution is to get rid of Typedef entirely and just define composite types directly in the functions that use them; that might result in some duplication, but I suspect that usually a given type isn't going to be deep enough that it would matter.