HMAC "Bitcoin Seed" for BIP32
It is specified in BIP32 (https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki). The SHA512-HMAC function is reused because it is already part of the standard elsewhere, but it takes a key in addition to the data being hashed. As the key can be arbitrary, we opted to use to make sure the key derivation was Bitcoin-specific.