Embed capnp.Client in capabilities.

[Louis Thibault]

Mentioned in the commit message, but: these generated wrappers are necessary for capabilities but not for structs, I think because the generated struct types embed capnp.Struct, whereas the interface types have an explicitly named field called Client. I don't seen an obvious reason why these should be different, so maybe we should just change this so that Client is embedded as well? I'm inclined to suggest that as a separate PR though, to keep things orthogonal.

Originally posted by @zenhack in #274 (comment)