[Feature] BIP 39 mnemonic seeds

[igorcafe]

Functions like mnemonic_from_seed and seed_from_mnemonic would be pretty cool :)

Maybe this can be useful: https://github.com/numsu/nanocurrency-web-js/blob/1134d96e28baa4096bfa63e8ae44fe6985993df6/lib/bip39-mnemonic.ts#L73