Move FROST to HashMaps

Honestly, the borrowed keys are frustrating, and this probably reduces 
performance while no longer offering an order when iterating. That said, 
they enable full u16 indexing and should mildly improve the API.

Cleans the Proof of Knowledge handling present in key gen.

coins/monero/src/tests/clsag.rs

@@ -1,5 +1,5 @@
 #[cfg(feature = "multisig")]
-use std::{cell::RefCell, rc::Rc};
+use std::{cell::RefCell, rc::Rc, collections::HashMap};
 
 use rand::{RngCore, rngs::OsRng};
 
@@ -71,7 +71,6 @@ fn clsag() {
 #[test]
 fn clsag_multisig() -> Result<(), MultisigError> {
   let (keys, group_private) = generate_keys();
-  let t = keys[0].params().t();
 
   let randomness = random_scalar(&mut OsRng);
   let mut ring = vec![];
@@ -92,9 +91,10 @@ fn clsag_multisig() -> Result<(), MultisigError> {
   }
 
   let mask_sum = random_scalar(&mut OsRng);
-  let mut machines = Vec::with_capacity(t);
-  for i in 1 ..= t {
-    machines.push(
+  let mut machines = HashMap::new();
+  for i in 1 ..= THRESHOLD {
+    machines.insert(
+      i,
       sign::AlgorithmMachine::new(
         ClsagMultisig::new(
           Transcript::new(b"Monero Serai CLSAG Test".to_vec()),
@@ -112,15 +112,15 @@ fn clsag_multisig() -> Result<(), MultisigError> {
             )
           )))
         ).unwrap(),
-        Rc::new(keys[i - 1].clone()),
-        &(1 ..= THRESHOLD).collect::<Vec<usize>>()
+        Rc::new(keys[&i].clone()),
+        &(1 ..= THRESHOLD).collect::<Vec<_>>()
       ).unwrap()
     );
   }
 
   let mut signatures = sign(&mut machines, &[1; 32]);
   let signature = signatures.swap_remove(0);
-  for s in 0 .. (t - 1) {
+  for s in 0 .. usize::from(THRESHOLD - 1) {
     // Verify the commitments and the non-decoy s scalar are identical to every other signature
     // FROST will already have called verify on the produced signature, before checking individual
     // key shares. For FROST Schnorr, it's cheaper. For CLSAG, it may be more expensive? Yet it