Use the serai_abi::Call in the actual Transaction type

We prior required they had the same encoding, yet this ensures they do by
making them one and the same. This does require an large, ugly, From/TryInto
block which is deemed preferable for moving this more and more into syntax
(from semantics).

Further improvements (notably re: Extra) is possible, and this already lets us
strip some members from the Call enum.

coordinator/src/tributary/scanner.rs

@@ -133,7 +133,13 @@ mod impl_pst_for_serai {
       key_pair: KeyPair,
       signature: Signature,
     ) {
-      let tx = SeraiValidatorSets::set_keys(set.network, removed, key_pair, signature);
+      // TODO: BoundedVec as an arg to avoid this expect
+      let tx = SeraiValidatorSets::set_keys(
+        set.network,
+        removed.try_into().expect("removing more than allowed"),
+        key_pair,
+        signature,
+      );
       async fn check(serai: SeraiValidatorSets<'_>, set: ValidatorSet, (): ()) -> bool {
         if matches!(serai.keys(set).await, Ok(Some(_))) {
           log::info!("another coordinator set key pair for {:?}", set);