use the version-handling code everywhere, and special case Text and Data copies for speed.
use the version-handling code everywhere, and special case Text and Data copies for speed.

zombiezen/go-capnproto2,zombiezen/go-capnproto2,zombiezen/go-capnproto,zombiezen/go-capnproto,hodduc/go-capnproto2,tpukep/caps,hodduc/go-capnproto2