Implement a proper Monero Timelock type

Transaction scanning now returns the timelock to ensure it's 
acknowledged by wallets.

Fixes https://github.com/serai-dex/serai/issues/16.

coins/monero/src/wallet/scan.rs

@@ -11,7 +11,7 @@ use monero::{consensus::deserialize, blockdata::transaction::ExtraField};
 use crate::{
   Commitment,
   serialize::{write_varint, read_32, read_scalar, read_point},
-  transaction::Transaction,
+  transaction::{Timelock, Transaction},
   wallet::{uniqueness, shared_key, amount_decryption, commitment_mask}
 };
 
@@ -57,13 +57,7 @@ impl Transaction {
     &self,
     view: Scalar,
     spend: EdwardsPoint
-  ) -> Vec<SpendableOutput> {
-    // Ignore transactions which utilize a timelock. Almost no transactions on Monero do,
-    // and they're not worth the effort to track given their complexities
-    if self.prefix.unlock_time != 0 {
-      return vec![];
-    }
-
+  ) -> (Vec<SpendableOutput>, Timelock) {
     let mut extra = vec![];
     write_varint(&u64::try_from(self.prefix.extra.len()).unwrap(), &mut extra).unwrap();
     extra.extend(&self.prefix.extra);
@@ -81,7 +75,7 @@ impl Transaction {
 
       pubkeys = m_pubkeys.iter().map(|key| key.point.decompress()).filter_map(|key| key).collect();
     } else {
-      return vec![];
+      return (vec![], self.prefix.timelock);
     };
 
     let mut res = vec![];
@@ -136,6 +130,7 @@ impl Transaction {
         }
       }
     }
-    res
+
+    (res, self.prefix.timelock)
   }
 }

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

@@ -27,7 +27,7 @@ use crate::{
     bulletproofs::Bulletproofs,
     RctBase, RctPrunable, RctSignatures
   },
-  transaction::{Input, Output, TransactionPrefix, Transaction},
+  transaction::{Input, Output, Timelock, TransactionPrefix, Transaction},
   rpc::{Rpc, RpcError},
   wallet::{SpendableOutput, Decoys, key_image_sort, uniqueness, shared_key, commitment_mask, amount_encryption}
 };
@@ -255,7 +255,7 @@ impl SignableTransaction {
     Transaction {
       prefix: TransactionPrefix {
         version: 2,
-        unlock_time: 0,
+        timelock: Timelock::None,
         inputs: vec![],
         outputs: tx_outputs,
         extra