Implement a FROST variant of Schnorrkel (#274)

* Minor lint

* Update frost-schnorrkel to the latest modular-frost

* Tidy up the schnorrkel library

crypto/schnorrkel/src/tests.rs

@@ -0,0 +1,25 @@
+use rand_core::OsRng;
+
+use group::GroupEncoding;
+use frost::{
+  Participant,
+  tests::{key_gen, algorithm_machines, sign},
+};
+
+use schnorrkel::{keys::PublicKey, context::SigningContext};
+
+use crate::Schnorrkel;
+
+#[test]
+fn test() {
+  const CONTEXT: &[u8] = b"FROST Schnorrkel Test";
+  const MSG: &[u8] = b"Hello, World!";
+
+  let keys = key_gen(&mut OsRng);
+  let key = keys[&Participant::new(1).unwrap()].group_key();
+  let machines = algorithm_machines(&mut OsRng, Schnorrkel::new(CONTEXT), &keys);
+  let signature = sign(&mut OsRng, Schnorrkel::new(CONTEXT), keys, machines, MSG);
+
+  let key = PublicKey::from_bytes(key.to_bytes().as_ref()).unwrap();
+  key.verify(&mut SigningContext::new(CONTEXT).bytes(MSG), &signature).unwrap()
+}