Add an IPNS code

[Amxx]

IPFS addresses are good enough to point to static resources, but the corresponding data cannot be updated (the update changes the resource hash, and thus gives a new IPFS address by design.

This issue is solved by IPNS, which are like tags pointing to an IPFS file. IPNS can be updated, which make them a great anchor to a website (for example). Yet IPNS is not listed in the codecs.

Adding a code for ipns-ns (0xe5?) would allow new synergies for easily upgradeable decentralized UIs. The current alternative (frequently updating an ENS entry) is expensive.