refactor: Relocate message handler interface (#543)

The MulticallHandler contract currently imports the SpokePool contract
and experienced a bytecode change after the following unrelated commit:

86593c6

This resulted in its CREATE2 deployment address changing. The 
MulticallHandler doesn't actually need any of the SpokePool
implementation or its dependencies, and it seems like we'd like the 
MulticallHandler to be deployed to the same address on as many chains as
possible, so factor out the interface to a separate file to insulate it
from any unintended modifications

Author: pxrl

contracts/SpokePool.sol

@@ -3,6 +3,7 @@ pragma solidity ^0.8.0;
 
 import "./MerkleLib.sol";
 import "./external/interfaces/WETH9Interface.sol";
+import "./interfaces/SpokePoolMessageHandler.sol";
 import "./interfaces/SpokePoolInterface.sol";
 import "./interfaces/V3SpokePoolInterface.sol";
 import "./upgradeable/MultiCallerUpgradeable.sol";
@@ -16,16 +17,6 @@ import "@openzeppelin/contracts-upgradeable/proxy/utils/UUPSUpgradeable.sol";
 import "@openzeppelin/contracts-upgradeable/security/ReentrancyGuardUpgradeable.sol";
 import "@openzeppelin/contracts/utils/math/SignedMath.sol";
 
-// This interface is expected to be implemented by any contract that expects to receive messages from the SpokePool.
-interface AcrossMessageHandler {
-    function handleV3AcrossMessage(
-        address tokenSent,
-        uint256 amount,
-        address relayer,
-        bytes memory message
-    ) external;
-}
-
 /**
  * @title SpokePool
  * @notice Base contract deployed on source and destination chains enabling depositors to transfer assets from source to

contracts/handlers/MulticallHandler.sol

@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: GPL-3.0-only
 pragma solidity ^0.8.0;
 
-import "../SpokePool.sol";
+import "../interfaces/SpokePoolMessageHandler.sol";
 import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
 import "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";
 import "@openzeppelin/contracts/utils/Address.sol";

contracts/interfaces/SpokePoolMessageHandler.sol

@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: BUSL-1.1
+pragma solidity ^0.8.0;
+
+// This interface is expected to be implemented by any contract that expects to receive messages from the SpokePool.
+interface AcrossMessageHandler {
+    function handleV3AcrossMessage(
+        address tokenSent,
+        uint256 amount,
+        address relayer,
+        bytes memory message
+    ) external;
+}