BIP39 support for ECDSA

[jasny]

Support BIP39 for generating private keys. For ECDSA, BIP39 should be the only method available for seeding.

https://levelup.gitconnected.com/blockchain-series-how-metamask-creates-accounts-a8971b21a74b

The current keyPairFromSeed method should be removed. Create a new class Bip39ECDSA, specifically for seeding.

To test if the implementation is correct

• Generate a new account using MetaMask (or another Ethereum wallet that uses BEP39)
• Get the 12 (or 24) word seed
• Use the seed to generate an ECDSA keypair using Seasalt
• Convert the public key into an ethereum address. See docs.

  You get a public address for your account by taking the last 20 bytes of the Keccak-256 hash of the public key and adding 0x to the beginning.

• The address should be the same address as shown in MetaMask