Derivation scheme for P2WPKH based accounts.

Sets the key layout wallets use for native segwit addresses, so any compatible wallet can find them from one backup.

A wallet makes all its keys from one seed, but must know which branch of its key tree to look in. This BIP reserves branch 84 for native segwit addresses (starting 'bc1q'), so restoring the same backup in another compatible wallet finds the same coins.