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/frost.rs

@@ -28,9 +28,9 @@ pub enum MultisigError {
   #[error("internal error ({0})")]
   InternalError(String),
   #[error("invalid discrete log equality proof")]
-  InvalidDLEqProof(usize),
+  InvalidDLEqProof(u16),
   #[error("invalid key image {0}")]
-  InvalidKeyImage(usize)
+  InvalidKeyImage(u16)
 }
 
 #[derive(Clone, Copy, PartialEq, Eq, Debug)]
@@ -168,7 +168,7 @@ impl DLEqProof {
   pub fn verify(
     &self,
     H: &DPoint,
-    l: usize,
+    l: u16,
     xG: &DPoint,
     xH: &DPoint
   ) -> Result<(), MultisigError> {
@@ -214,7 +214,7 @@ pub fn read_dleq(
   serialized: &[u8],
   start: usize,
   H: &DPoint,
-  l: usize,
+  l: u16,
   xG: &DPoint
 ) -> Result<dfg::EdwardsPoint, MultisigError> {
   // Not using G_from_slice here would enable non-canonical points and break blame