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

Cargo.lock

@@ -1614,7 +1614,7 @@ dependencies = [
 
 [[package]]
 name = "dleq"
-version = "0.1.1"
+version = "0.1.2"
 dependencies = [
  "blake2",
  "dalek-ff-group",
@@ -4507,7 +4507,7 @@ dependencies = [
 
 [[package]]
 name = "modular-frost"
-version = "0.2.4"
+version = "0.3.0"
 dependencies = [
  "dalek-ff-group",
  "dleq",