Fold BIP39 mnemonic-based keypair generation into solana-keygen

[mvines]

Let's bring the functionality of https://github.com/fkbenjamin/solana_keygen into solana-keygen:

1. The ability to output a BIP39 mnemonic for a keypair
2. The ability to recover a keypair from a BIP39 mnemonic

[garious]

Please consider HD wallet key generation while in the neighborhood: #5246

[jstarry]

@mvines Doesn't seem like #1 is doable, afaik you cannot recover a mnemonic seed phrase from a generated keypair. What we can do is optionally show a user their mnemonic seed phrase when generating a new keypair. Sound good?

[mvines]

What we can do is optionally show a user their mnemonic seed phrase when generating a new keypair

Yep!

[jstarry]

@mvines @garious it feels a bit strange to implement BIP39 without BIP32..

Without BIP32 we decide our own way of deriving keypairs from a mnemonic phrase. For example, in #5889 I derive a keypair secret key from the first 32 bytes of the mnemonic seed. If we support BIP32 later on, we would then have to support two different ways of deriving keypairs.

Any recommendations on how to proceed here?

[mvines]

I'm ok with changing the solana-keygen user interface later when we add BIP32 (cc: #5246)