Use a transcript when generating the per-chain binding for a given set of keys

While it was fine as-is, as it only had one variable length property, 
this is a bit more robust. Also binds the Curve ID, which should declare 
differently even for just different basepoints, and therefore adds two 
variable length properties (justifying the transcript).

coins/monero/src/frost.rs

@@ -135,7 +135,7 @@ impl DLEqProof {
     // the proper order if they want to reach consensus
     // It'd be a poor API to have CLSAG define a new transcript solely to pass here, just to try to
     // merge later in some form, when it should instead just merge xH (as it does)
-    let mut transcript = Transcript::new(b"DLEq Proof".to_vec());
+    let mut transcript = Transcript::new(b"DLEq Proof");
     // Bit redundant, keeps things consistent
     transcript.domain_separate(b"DLEq");
     // Doesn't include G which is constant, does include H which isn't, even though H manipulation

coins/monero/src/tests/clsag.rs

@@ -96,7 +96,7 @@ fn clsag_multisig() -> Result<(), MultisigError> {
     algorithm_machines(
       &mut OsRng,
       ClsagMultisig::new(
-        Transcript::new(b"Monero Serai CLSAG Test".to_vec()),
+        Transcript::new(b"Monero Serai CLSAG Test"),
         Rc::new(RefCell::new(Some(
           ClsagDetails::new(
             ClsagInput::new(

coins/monero/src/wallet/send/multisig.rs

@@ -37,7 +37,7 @@ pub struct TransactionMachine {
 impl SignableTransaction {
   pub async fn multisig<R: RngCore + CryptoRng>(
     mut self,
-    label: Vec<u8>,
+    mut transcript: Transcript,
     rng: &mut R,
     rpc: &Rpc,
     height: usize,
@@ -56,8 +56,9 @@ impl SignableTransaction {
     // Create a RNG out of the input shared keys, which either requires the view key or being every
     // sender, and the payments (address and amount), which a passive adversary may be able to know
     // depending on how these transactions are coordinated
+    // Being every sender would already let you note rings which happen to use your transactions
+    // multiple times, already breaking privacy there
 
-    let mut transcript = Transcript::new(label);
     transcript.domain_separate(b"monero_transaction");
     // Include the height we're using for our data
     // The data itself will be included, making this unnecessary, yet a lot of this is technically

coins/monero/tests/send.rs

@@ -25,7 +25,7 @@ mod rpc;
 use crate::rpc::{rpc, mine_block};
 
 #[cfg(feature = "multisig")]
-use monero_serai::frost::Ed25519;
+use monero_serai::frost::{Transcript, Ed25519};
 
 lazy_static! {
   static ref SEQUENTIAL: Mutex<()> = Mutex::new(());
@@ -145,7 +145,7 @@ async fn send_core(test: usize, multisig: bool) {
           machines.insert(
             i,
             signable.clone().multisig(
-              b"Monero Serai Test Transaction".to_vec(),
+              Transcript::new(b"Monero Serai Test Transaction"),
               &mut OsRng,
               &rpc,
               rpc.get_height().await.unwrap() - 10,