Remove sender_i from DkgShares

It was a piece of duplicated data used to achieve context-less
de)serialization. This new Vec code is a bit tricker to first read, yet overall
clean and removes a potential fault.

Saves 2 bytes from DkgShares messages.

coordinator/src/tests/tributary/mod.rs

@@ -1,10 +1,7 @@
 use core::fmt::Debug;
-use std::collections::HashMap;
 
 use rand_core::{RngCore, OsRng};
 
-use frost::Participant;
-
 use tributary::{ReadWrite, tests::random_signed};
 
 use crate::tributary::{SignData, Transaction};
@@ -20,10 +17,6 @@ mod dkg;
 mod handle_p2p;
 mod sync;
 
-fn random_u16<R: RngCore>(rng: &mut R) -> u16 {
-  u16::try_from(rng.next_u64() >> 48).unwrap()
-}
-
 fn random_u32<R: RngCore>(rng: &mut R) -> u32 {
   u32::try_from(rng.next_u64() >> 32).unwrap()
 }
@@ -70,18 +63,17 @@ fn serialize_transaction() {
     // This supports a variable share length, yet share length is expected to be constant among
     // shares
     let share_len = usize::try_from(OsRng.next_u64() % 512).unwrap();
-    // Create a valid map of shares
-    let mut shares = HashMap::new();
+    // Create a valid vec of shares
+    let mut shares = vec![];
     // Create up to 512 participants
     for i in 0 .. (OsRng.next_u64() % 512) {
       let mut share = vec![0; share_len];
       OsRng.fill_bytes(&mut share);
-      shares.insert(Participant::new(u16::try_from(i + 1).unwrap()).unwrap(), share);
+      shares.push(share);
     }
 
     test_read_write(Transaction::DkgShares {
       attempt: random_u32(&mut OsRng),
-      sender_i: Participant::new(random_u16(&mut OsRng).saturating_add(1)).unwrap(),
       shares,
       confirmation_nonces: {
         let mut nonces = [0; 64];