Remove Output::amount and move Payment from Amount to Balance

This code is still largely designed around the idea a payment for a network is
fungible with any other, which isn't true. This starts moving past that.

Asserts are added to ensure the integrity of coin to the scheduler (which is
now per key per coin, not per key alone) and in Bitcoin/Monero prepare_send.

processor/src/multisigs/db.rs

@@ -5,10 +5,9 @@ use ciphersuite::Ciphersuite;
 pub use serai_db::*;
 
 use scale::{Encode, Decode};
-#[rustfmt::skip]
 use serai_client::{
-  primitives::ExternalAddress,
-  in_instructions::primitives::InInstructionWithBalance
+  primitives::{Balance, ExternalAddress},
+  in_instructions::primitives::InInstructionWithBalance,
 };
 
 use crate::{
@@ -172,23 +171,23 @@ impl<N: Network, D: Db> MultisigsDb<N, D> {
     res
   }
 
-  fn forwarded_output_key(amount: u64) -> Vec<u8> {
-    Self::multisigs_key(b"forwarded_output", amount.to_le_bytes())
+  fn forwarded_output_key(balance: Balance) -> Vec<u8> {
+    Self::multisigs_key(b"forwarded_output", balance.encode())
   }
   pub fn save_forwarded_output(
     txn: &mut D::Transaction<'_>,
     instruction: InInstructionWithBalance,
   ) {
-    let key = Self::forwarded_output_key(instruction.balance.amount.0);
+    let key = Self::forwarded_output_key(instruction.balance);
     let mut existing = txn.get(&key).unwrap_or(vec![]);
     existing.extend(instruction.encode());
     txn.put(key, existing);
   }
   pub fn take_forwarded_output(
     txn: &mut D::Transaction<'_>,
-    amount: u64,
+    balance: Balance,
   ) -> Option<InInstructionWithBalance> {
-    let key = Self::forwarded_output_key(amount);
+    let key = Self::forwarded_output_key(balance);
 
     let outputs = txn.get(&key)?;
     let mut outputs_ref = outputs.as_slice();