Expose type IDs in the generated code somehow.

[zenhack]

It's sometimes useful to be able to get at the type IDs for structs et al. We should facilitate this somehow. I'm thinking a type class:

class TypeID a where
    typeID :: Proxy a -> Word64

...plus generated instances.

[zenhack]

Note that I've been moving the library in the direction of using TypeApplications instead of Proxy. But we should still do this at some point.