goprotobuf: Use encoding.TextMarshaler and encoding.TextUnmarshaler in place of
the identical custom interfaces.

encoding.Text{Marshaler,Unmarshaler} were added in Go 1.2 (April 2013);
it's safe to rely on them now.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/181770043
2 files changed