This change implements ICMP multi-part message marshaler, parser and
extensions for MPLS which are used for route trace applications as
described in RFC 4950.

API breaking changes:

type MessageBody interface, Len() int
type Extension interface, Len() int
type Extension interface, Marshal() ([]byte, error)

are replaced with

type MessageBody interface, Len(int) int
type Extension interface, Len(int) int
type Extension interface, Marshal(int) ([]byte, error)

Change-Id: Iee1f2e03916d49b8dfe3a89fe682c702d40ecc85
Reviewed-on: https://go-review.googlesource.com/2794Reviewed-by: Ian Lance Taylor <iant@golang.org>