Refactor archive to be sequence of pairs

[gordonbrander]

This PR refactors the archive format to prioritize streaming encoding and decoding over random access.

TODO:

• Streaming unarchiving
• Add content-type to file memo headers (useful for serving from archives)
• Add iss-nickname to headers
• Remove "default" key name. Get user to choose name for key on generation.
• TOFU (Trust On First Use). Prompt user to trust (add to address book) new keys. Register them via their nickname by default.

Advantages:

• Makes archives useful for more than just files... general purpose bundling.
• Allows concatenating onto archives, since each memo is signed independently.
• Supports archives from multiple sources
• Blake3/Bao/random access are better served via a pass that derives a sidecar index, such as a Bao file
  • E.g. in a scenario where you want to support cheap random access via range requests, a Bao and range index could be generated in a single pass on first request. Following requests defer to the existing index.

Tradeoffs:

• No signature over total set of memos (this is in tension with concatenation). However, this could be bolted on by generating and signing a HashSeq memo as the first item in the sequence, if desired.

[gordonbrander]

Fixes #24