Use rustpkg conventions

[chris-morgan]

It'd be good to be able to use this with rustpkg. The most notable change is that capnprust.rs should be renamed to lib.rs, but there may be other changes necessary; I haven't delved into this project's compiler stuff and do not know if it needs anything special. Tests also get done with test.rs being the root file in a crate.

[dwrensha]

I'm using the "lib.rs" and "main.rs" convention now. I didn't figure out how to actually get installation working with rustpkg, but that's irrelevant now that it's gone: rust-lang/rust#11987