Implement TX creation

Updates CLSAG signing as needed. Moves around Error types.

CLSAG multisig and the multisig feature is currently completely borked 
because of this. The created TXs are accepted by Monero nodes.

coins/monero/src/frost.rs

@@ -1,6 +1,7 @@
 use core::convert::TryInto;
 
 use rand_core::{RngCore, CryptoRng};
+use thiserror::Error;
 
 use blake2::{digest::Update, Digest, Blake2b512};
 
@@ -19,7 +20,17 @@ use group::Group;
 use dalek_ff_group as dfg;
 use frost::{CurveError, Curve};
 
-use crate::{SignError, random_scalar};
+use crate::random_scalar;
+
+#[derive(Error, Debug)]
+pub enum MultisigError {
+  #[error("internal error ({0})")]
+  InternalError(String),
+  #[error("invalid discrete log equality proof")]
+  InvalidDLEqProof,
+  #[error("invalid key image {0}")]
+  InvalidKeyImage(usize)
+}
 
 #[derive(Clone, Copy, PartialEq, Eq, Debug)]
 pub struct Ed25519;
@@ -137,7 +148,7 @@ impl DLEqProof {
     H: &DPoint,
     primary: &DPoint,
     alt: &DPoint
-  ) -> Result<(), SignError> {
+) -> Result<(), MultisigError> {
     let s = self.s;
     let c = self.c;
 
@@ -154,7 +165,7 @@ impl DLEqProof {
 
     // Take the opportunity to ensure a lack of torsion in key images/randomness commitments
     if (!primary.is_torsion_free()) || (!alt.is_torsion_free()) || (c != expected_c) {
-      Err(SignError::InvalidDLEqProof)?;
+      Err(MultisigError::InvalidDLEqProof)?;
     }
 
     Ok(())