/// An output to a P2WPKH, spendable exclusively by the given private key.
/// The witness in the spending input, is, thus, simply:
/// <BIP 143 signature generated with the given key> <public key derived from the given key>
/// An output to a P2WPKH, spendable exclusively by the given private key.
/// The witness in the spending input, is, thus, simply:
/// <BIP 143 signature generated with the given key> <public key derived from the given key>