asn1: Improved encoding/decoding for OID types

The first identifier in an Object Identifer must be between 0 and 2
inclusive. The range of values that the second one can take depends
on the value of the first one.
The two first identifiers are not necessarily encoded in a single octet,
but in a varint.

R=golang-dev, agl
CC=golang-dev
https://golang.org/cl/10140046