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/plan.rs

@@ -1,17 +1,20 @@
 use std::io;
 
+use scale::{Encode, Decode};
+
 use transcript::{Transcript, RecommendedTranscript};
 use ciphersuite::group::GroupEncoding;
 use frost::curve::Ciphersuite;
 
+use serai_client::primitives::Balance;
+
 use crate::networks::{Output, Network};
 
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub struct Payment<N: Network> {
   pub address: N::Address,
   pub data: Option<Vec<u8>>,
-  // TODO: Balance
-  pub amount: u64,
+  pub balance: Balance,
 }
 
 impl<N: Network> Payment<N> {
@@ -21,7 +24,8 @@ impl<N: Network> Payment<N> {
     if let Some(data) = self.data.as_ref() {
       transcript.append_message(b"data", data);
     }
-    transcript.append_message(b"amount", self.amount.to_le_bytes());
+    transcript.append_message(b"coin", self.balance.coin.encode());
+    transcript.append_message(b"amount", self.balance.amount.0.to_le_bytes());
   }
 
   pub fn write<W: io::Write>(&self, writer: &mut W) -> io::Result<()> {
@@ -40,7 +44,7 @@ impl<N: Network> Payment<N> {
       writer.write_all(data)?;
     }
 
-    writer.write_all(&self.amount.to_le_bytes())
+    writer.write_all(&self.balance.encode())
   }
 
   pub fn read<R: io::Read>(reader: &mut R) -> io::Result<Self> {
@@ -63,11 +67,10 @@ impl<N: Network> Payment<N> {
       None
     };
 
-    let mut buf = [0; 8];
-    reader.read_exact(&mut buf)?;
-    let amount = u64::from_le_bytes(buf);
+    let balance = Balance::decode(&mut scale::IoReader(reader))
+      .map_err(|_| io::Error::new(io::ErrorKind::Other, "invalid balance"))?;
 
-    Ok(Payment { address, data, amount })
+    Ok(Payment { address, data, balance })
   }
 }