encoding/ianaindex: first implementation

Note that Get was renamed to the more idiomatic Encoding.
Added a MIB index for MIB naming.

Fixes golang/go#18278.

Change-Id: I421e9afe3be1a759c034e2e0a4659511f437aeea
Reviewed-on: https://go-review.googlesource.com/36515
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
5 files changed