Enforce FROST StateMachine progression via the type system

A comment on the matter was made in 
https://github.com/serai-dex/serai/issues/12. While I do believe the API 
is slightly worse, I appreciate the explicitness.

processor/src/coins/monero.rs

@@ -1,7 +1,6 @@
 use std::sync::Arc;
 
 use async_trait::async_trait;
-use rand_core::OsRng;
 
 use curve25519_dalek::{constants::ED25519_BASEPOINT_TABLE, scalar::Scalar};