rename namespaces and remove "ipfs" alias for "p2p"

[Stebalien]

Previously, we called these foo-ns. Now we just call them foo (the fact that they're "namespaces" is implicit.

This way, the path component always matches the multicodec name (for both multiaddr and other path types).

[ben221199]

Nice.

[lidel]

cc @makoto – fysa in case this impacts ENS (i know you use ipfs-ns and ipns-ns codes for identifying /ipfs/ and /ipns/ content paths, but not sure if you ever rely on their string labels)

[guseggert]

I would like to use human-readable codec names in an API but this makes me think twice, is it incorrect to consider these names as stable?

If these are considered unstable, then we should make this clear in the docs and probably remove the recommendation to parse these names or use them programmatically.

[rvagg]

@guseggert mostly we try to keep them stable, but there are occasions like this when they get changed but I think in this case these names are not as likely to be used as they were anyway so the impact is considered fairly low. We're not about to go changing "dag-pb" or other commonly used names, however, even in cases where there's name-regret.

We also have this "draft" thing now, and these are drafts so that lessens the burden. Perhaps we don't document it but maybe there's an implicit "we won't touch this if it's 'permanent'".