(Unconstrained) AnyPointer constants.
(Unconstrained) AnyPointer constants.

I'm going to say this closes #41; there are a few types that we still
don't generate constants for, but:

1. Interfaces barely make sense as constants, and will have to wait for
   RPC support anyway.
2. Group constants don't make a ton of sense to me either. Is it even
   possible to encode these in the schema's text form?
3. Constrained AnyPointers need to wait for type parameters (#29).

zenhack/haskell-capnp