Create dedicated message structures for FROST messages (#140)

* Create message types for FROST key gen

Taking in reader borrows absolutely wasn't feasible. Now, proper types
which can be read (and then passed directly, without a mutable borrow)
exist for key_gen. sign coming next.

* Move FROST signing to messages, not Readers/Writers/Vec<u8>

Also takes the nonce handling code and makes a dedicated file for it, 
aiming to resolve complex types and make the code more legible by 
replacing its previously inlined state.

* clippy

* Update FROST tests

* read_signature_share

* Update the Monero library to the new FROST packages

* Update processor to latest FROST

* Tweaks to terminology and documentation

crypto/frost/src/promote.rs

@@ -12,10 +12,7 @@ use group::GroupEncoding;
 use transcript::{Transcript, RecommendedTranscript};
 use dleq::DLEqProof;
 
-use crate::{
-  curve::{CurveError, Curve},
-  FrostError, FrostCore, FrostKeys, validate_map,
-};
+use crate::{curve::Curve, FrostError, FrostCore, FrostKeys, validate_map};
 
 /// Promote a set of keys to another Curve definition.
 pub trait CurvePromote<C2: Curve> {
@@ -73,11 +70,8 @@ impl<C: Curve> GeneratorProof<C> {
     self.proof.serialize(writer)
   }
 
-  pub fn deserialize<R: Read>(reader: &mut R) -> Result<GeneratorProof<C>, CurveError> {
-    Ok(GeneratorProof {
-      share: C::read_G(reader)?,
-      proof: DLEqProof::deserialize(reader).map_err(|_| CurveError::InvalidScalar)?,
-    })
+  pub fn deserialize<R: Read>(reader: &mut R) -> io::Result<GeneratorProof<C>> {
+    Ok(GeneratorProof { share: C::read_G(reader)?, proof: DLEqProof::deserialize(reader)? })
   }
 }