Implement ed25519 primitives for proposals 220, 224, 228

For proposal 220, we need ed25519 with detached signatures. (legacy/trac#12498 (moved))

For proposal 224 (legacy/trac#12424 (moved)), we need ed25519 with key blinding (legacy/trac#8106 (moved)).

For proposal 228, we need the ability to convert curve25519 keys to ed25519 keys. (legacy/trac#12499 (moved))