Update the Router smart contract to pay fees to the caller

The caller is paid a fixed fee per unit of gas spent. That arguably
incentivizes the publisher to raise the gas used by internal calls, yet this
doesn't effect the user UX as they'll have flatly paid the worst-case fee
already. It does pose a risk where callers are arguably incentivized to cause
transaction failures which consume all the gas, not just increased gas, yet:

1) Modern smart contracts don't error by consuming all the gas
2) This is presumably infeasible
3) Even if it was feasible, the gas fees gained presumably exceed the gas fees
   spent causing the failure

The benefit to only paying the callers for the gas used, not the gas alotted,
is it allows Serai to build up a buffer. While this should be minor, a few
cents on every transaction at best, if we ever do have any costs slip through
the cracks, it ideally is sufficient to handle those.

processor/ethereum/router/src/lib.rs

@@ -47,7 +47,7 @@ impl From<&Signature> for abi::Signature {
 }
 
 /// A coin on Ethereum.
-#[derive(Clone, PartialEq, Eq, Debug)]
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
 pub enum Coin {
   /// Ether, the native coin of Ethereum.
   Ether,
@@ -56,6 +56,14 @@ pub enum Coin {
 }
 
 impl Coin {
+  fn address(&self) -> Address {
+    (match self {
+      Coin::Ether => [0; 20],
+      Coin::Erc20(address) => *address,
+    })
+    .into()
+  }
+
   /// Read a `Coin`.
   pub fn read<R: io::Read>(reader: &mut R) -> io::Result<Self> {
     let mut kind = [0xff];
@@ -152,12 +160,12 @@ impl InInstruction {
 /// A list of `OutInstruction`s.
 #[derive(Clone)]
 pub struct OutInstructions(Vec<abi::OutInstruction>);
-impl From<&[(SeraiAddress, (Coin, U256))]> for OutInstructions {
-  fn from(outs: &[(SeraiAddress, (Coin, U256))]) -> Self {
+impl From<&[(SeraiAddress, U256)]> for OutInstructions {
+  fn from(outs: &[(SeraiAddress, U256)]) -> Self {
     Self(
       outs
         .iter()
-        .map(|(address, (coin, amount))| {
+        .map(|(address, amount)| {
           #[allow(non_snake_case)]
           let (destinationType, destination) = match address {
             SeraiAddress::Address(address) => (
@@ -166,19 +174,14 @@ impl From<&[(SeraiAddress, (Coin, U256))]> for OutInstructions {
             ),
             SeraiAddress::Contract(contract) => (
               abi::DestinationType::Code,
-              (abi::CodeDestination { gas: contract.gas(), code: contract.code().to_vec().into() })
-                .abi_encode(),
+              (abi::CodeDestination {
+                gas_limit: contract.gas_limit(),
+                code: contract.code().to_vec().into(),
+              })
+              .abi_encode(),
             ),
           };
-          abi::OutInstruction {
-            destinationType,
-            destination: destination.into(),
-            coin: match coin {
-              Coin::Ether => [0; 20].into(),
-              Coin::Erc20(address) => address.into(),
-            },
-            value: *amount,
-          }
+          abi::OutInstruction { destinationType, destination: destination.into(), value: *amount }
         })
         .collect(),
     )
@@ -318,17 +321,31 @@ impl Router {
   }
 
   /// Get the message to be signed in order to execute a series of `OutInstruction`s.
-  pub fn execute_message(chain_id: U256, nonce: u64, outs: OutInstructions) -> Vec<u8> {
-    ("execute", chain_id, U256::try_from(nonce).expect("couldn't convert u64 to u256"), outs.0)
+  pub fn execute_message(
+    chain_id: U256,
+    nonce: u64,
+    coin: Coin,
+    fee_per_gas: U256,
+    outs: OutInstructions,
+  ) -> Vec<u8> {
+    ("execute", chain_id, U256::try_from(nonce).unwrap(), coin.address(), fee_per_gas, outs.0)
       .abi_encode()
   }
 
   /// Construct a transaction to execute a batch of `OutInstruction`s.
-  pub fn execute(&self, outs: OutInstructions, sig: &Signature) -> TxLegacy {
+  pub fn execute(
+    &self,
+    coin: Coin,
+    fee_per_gas: U256,
+    outs: OutInstructions,
+    sig: &Signature,
+  ) -> TxLegacy {
     let outs_len = outs.0.len();
     TxLegacy {
       to: TxKind::Call(self.1),
-      input: abi::executeCall::new((outs.0, sig.into())).abi_encode().into(),
+      input: abi::executeCall::new((coin.address(), fee_per_gas, outs.0, sig.into()))
+        .abi_encode()
+        .into(),
       // TODO
       gas_limit: 100_000 + ((200_000 + 10_000) * u128::try_from(outs_len).unwrap()),
       ..Default::default()