Split core schema output into a separate package

[zenhack]

The core schema modules (Capnp.Capnp.*) won't be needed by every user of the library, and they dominate the compile time for this project. The code generator only needs schema.capnp (and transitively c++.capnp but that's trivial). It would be nice to divide things up into a few separate packages so that users of the library don't have to build a whole bunch of code they won't use.