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/lib.rs

@@ -38,6 +38,7 @@ pub mod promote;
 
 /// Algorithm for the signing process.
 pub mod algorithm;
+mod nonce;
 /// Threshold signing protocol.
 pub mod sign;
 
@@ -45,7 +46,7 @@ pub mod sign;
 #[cfg(any(test, feature = "tests"))]
 pub mod tests;
 
-// Validate a map of serialized values to have the expected included participants
+// Validate a map of values to have the expected included participants
 pub(crate) fn validate_map<T>(
   map: &HashMap<u16, T>,
   included: &[u16],
@@ -136,6 +137,8 @@ pub enum FrostError {
   InvalidCommitment(u16),
   #[error("invalid proof of knowledge (participant {0})")]
   InvalidProofOfKnowledge(u16),
+  #[error("invalid preprocess (participant {0})")]
+  InvalidPreprocess(u16),
   #[error("invalid share (participant {0})")]
   InvalidShare(u16),