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.

crypto/frost/src/algorithm.rs

@@ -28,7 +28,7 @@ pub trait Algorithm<C: Curve>: Clone {
   fn process_addendum(
     &mut self,
     params: &MultisigView<C>,
-    l: usize,
+    l: u16,
     commitments: &[C::G; 2],
     serialized: &[u8],
   ) -> Result<(), FrostError>;
@@ -131,7 +131,7 @@ impl<C: Curve, H: Hram<C>> Algorithm<C> for Schnorr<C, H> {
   fn process_addendum(
     &mut self,
     _: &MultisigView<C>,
-    _: usize,
+    _: u16,
     _: &[C::G; 2],
     _: &[u8],
   ) -> Result<(), FrostError> {