Extension and format name

[wking]

Previous suggestions from ipfs/kubo#1195:

Merkle-ness:

• dar for merkleDag ARchive (clashes with Disk ARchive)
• mar for Merkle(dag) ARchive
• kar for merKledag ARchive

IPFS-ness:

• ipo for InterPlanetaryObjects

Self-certifying-ness:

• aar for Authenticated ARchive (pirates!)
• car for Certified ARchive
• sar for Secure ARchive
• yar for ______ ARchive

Other:

• oar for Object ARchive or Oort Archive
• ship for shipping things around (interplanetary travel?)

The main considerations from ipfs/kubo#1195 are:

• We'd rather not conflict with existing tools, but we can if we decide the conflict is worth it.
• Hard consonant sounds are popular.
• We'd like a strong connection to the IPFS-ness and/or the archive-ness.

I'm personally not interested in the self-certifying connection, because I want this to be a generic IPFS-object archive, and you need signature and public-key objects to be self-certifying.

[rht]

One more for "merkle"ness: .mag