Switch to Ed25519 for keys

Currently, ZeroNet uses ECDSA for its signatures. It would probably be a good idea to switch to Ed25519 long-term, for the following reasons:
- Ed25519 public keys are slightly shorter than ECDSA keys (33 vs 32 bytes, no 64-byte "uncompressed" public key format)
- Ed25519 is slightly easier to implement securely, because side channel attacks are harder to pull off, and because there is no need for a secure RNG to make signatures
- Ed25519 is what BitTorrent uses, so it would be beneficial for interoperability if that ever happens
- Using ECDSA in the Base58 encoding encourages unsafe kinds of key reuse

In principle, this is not an urgent change, and it would be breaking, so it's probably reasonable to do this at the same time as #2781.

No files, tests, or entry points are named. First clarify the migration and compatibility scope with issue #2781, then trace where ZeroNet currently creates, encodes, verifies, and interoperates with ECDSA keys; done means the breaking Ed25519 transition is specified across those paths.

索引モデルが issue の本文から書いたものです。