Spin off libraries kj-async and capnp-rpc, so that people who don't want them can avoid their large footprint.

Refactor capability code using fork.  Still too much refcounting, though.  Maybe this calls for a different design for pipelining...

Interface code generation.  Also update method grammar to allow multiple returns, and allow specifying request/response structs instead of parameter lists.

Moar type-safety, rewriting WireFormat to avoid descriptors (incomplete), decided struct pointers should point at the beginning of the struct after all.