Update prefix bytes encoding to include mh len

[austinabell]

Summary of changes
Changes introduced in this pull request:

• I actually have the worst memory, Cid Prefix to bytes #428 was misleading because it did exist and I had just removed the mh_len during the multihash refactor because it was redundant (at the time) so I added it back
  • Also figured I'd move prefix to own file because that file isn't setup the best (not going to do an actual refactor because hopefully we move to shared rust cid crate in future)

I assume the length is needed for variable length identity hashes, which is why it's included in the prefix bytes.

Reference issue to close (if applicable)

Closes #428

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