internal/catmsg: added encoders and decoders

Change-Id: I168f19c881ae8dcc4163b9ab96e3ea8cc06158f5
Reviewed-on: https://go-review.googlesource.com/41331
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
3 files changed