Define multicodec's prefix

[jmitchell]

Suppose I come across a multicodec-encoded stream in the wild, and have no context for it. After some investigating I might conclude everything after the first few bytes is a standard format. But what are those first few bytes?

If there were a registered prefix for multicodec itself, then all stored/transmitted multicodec streams could start with the same recognizable "magic number". Once the stream is recognized as a multicodec stream, people/machines will know to look at the prefix in the next few bytes to determine the nature of the payload.