Support working with ed25519 seeds in addition to raw keypairs.

[aqua]

This is the same as #1045 but for the dev branch, since the README requested PRs against dev.

Original PR text:

Addresses issue #1044.

• Stores the original seed during keygen
• Adds get prv.seed CLI to obtain it
• Adds set prv.seed to generate a new keypair from a seed chosen off-node (could be useful for devices without reliable entropy)
• Reworks Identity::writeTo() to semantically explicit forms since the seed and the pubkey are the same length and can't be disambiguated soley via the size_t.