capnpc-go: use more specific pointers for AnyStruct, AnyList, Capability

[zombiezen]

Cap'n Proto schemas can specify AnyStruct, AnyList, or Capability to limit what kind of pointers a field can have. In the generated code, these are all given capnp.Ptr types. The code generator should distinguish these and use the appropriate type.

[zenhack]

@lthibault has already done this for capabilities, and I think was planning on doing the rest, so I'm going to assign him.

[zenhack]

I guess technically said patch hasn't landed (#232)

[lthibault]

@zenhack Should I just go ahead and merge #232 since it doesn't seem to actually be introducing the race condition in the tests?

[zenhack]

I went ahead and merged it.