We should write threshold schnorr signatures using session types analogous to the multi-signature ones in multi.rs. We should first go over the threshold schnorr signing literature just to check if anything needs tweaking.

We'll likely need Pederson's DKG as a subprotocol for both shared key generation and for creating R. See https://pdfs.semanticscholar.org/642b/d1bbc86c7750cef9fa770e9e4ba86bd49eb9.pdf