Don't declare encoder if not used.
Don't declare encoder if not used.

If the encoder wasn't used in a function, the Go compiler would not compile the file.

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