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/transaction/mixins.rs

@@ -0,0 +1,15 @@
+// TOOD
+pub(crate) fn select(o: u64) -> (u8, Vec<u64>) {
+  let mut mixins: Vec<u64> = (o .. o + 11).into_iter().collect();
+  mixins.sort();
+  (0, mixins)
+}
+
+pub(crate) fn offset(mixins: &[u64]) -> Vec<u64> {
+  let mut res = vec![mixins[0]];
+  res.resize(11, 0);
+  for m in (1 .. mixins.len()).rev() {
+    res[m] = mixins[m] - mixins[m - 1];
+  }
+  res
+}