align better with KERI and provide better upgrade path for did:key

[dhh1128]

Provide a new numeric algorithm which is just the simplest self-certifying behavior that KERI has advocated, and which is essentially the same thing as did:key. This allows a did:key to be upgraded to a did:peer by simply rewriting its prefix, and to then have an upgrade path to a dynamic, updateable DID with endpoints and full DID doc support, by exchanging deltas.

[OR13]

Cross linking some historical context for future readers:

• https://github.com/decentralized-identity/didcomm-messaging/issues/33#issuecomment-592709282
• https://github.com/w3c-ccg/did-method-key/issues/5

Hoping one day we can do something similar with sidetree based methods :)

[dhh1128]

Enough time has elapsed now for comment on this change, and most of the people likely to have an opinion have reviewed and weighed in. All comments resolved. Proceeding with merge.