Use Monero-compatible additional TX keys

This still sends a fingerprinting flare up if you send to a subaddress which
needs to be fixed. Despite that, Monero no should no longer fail to scan TXs
from monero-serai regarding additional keys.

Previously it failed becuase we supplied one key as THE key, and n-1 as
additional. Monero expects n for additional.

This does correctly select when to use THE key versus when to use the additional
key when sending. That removes the ability for recipients to fingerprint
monero-serai by receiving to a standard address yet needing to use an additional
key.

coins/monero/src/wallet/mod.rs

@@ -54,12 +54,12 @@ pub(crate) fn uniqueness(inputs: &[Input]) -> [u8; 32] {
 #[allow(non_snake_case)]
 pub(crate) fn shared_key(
   uniqueness: Option<[u8; 32]>,
-  s: &Scalar,
+  s: &Zeroizing<Scalar>,
   P: &EdwardsPoint,
   o: usize,
 ) -> (u8, Scalar, [u8; 8]) {
   // 8Ra
-  let mut output_derivation = (s * P).mul_by_cofactor().compress().to_bytes().to_vec();
+  let mut output_derivation = (s.deref() * P).mul_by_cofactor().compress().to_bytes().to_vec();
 
   let mut payment_id_xor = [0; 8];
   payment_id_xor