Could we implement one-pass signing?

[sfluhrer]

The current code performs two passes over the message during the signing process. While this is required by the way Sphincs+ is currently defined, it is less than optimal if you are signing a large message. Could we implement a one-pass option during the signing process. Note that, in this mode, this would not be NIST compliant (or won't be, when NIST eventually approves it), and it will require randomness during the signature.