add bip38 (non-ec-multiply) support to Bitcoin::Key

  key = Bitcoin::Key.from_bip38(encrypted_privkey, passphrase)
  key.to_bip38(passphrase)
add warp support for Bitcoin::Key

  key = Bitcoin::Key.from_warp(passphrase, salt)
storage: implement simple #get-block_id_for_tx_id as default for all backends