Write this package

[jbenet]

It should have types and functions useful to manipulate multicodec-packed.

At least these, but probably other things:

// this represents a varint code that we can use in a table.
type Code []byte // maybe string to allow using it in maps

// some table type that maps a Code to a multicodec path
// so it can be used with the multicodec package.
type Table map[Code]string

func GetCode(buf []byte) Code {} // a way to retrieve the code from a value type.
func AddPrefix(Code, buf []byte) []byte {}  // add a multicodec-packed prefix
func RemovePrefix(buf []byte) []byte {} // remove the multicodec-packed prefix