Extract IFiatToken interface

contracts/tokenbridge/arbitrum/gateway/L2USDCGateway.sol

@@ -2,7 +2,7 @@
 pragma solidity ^0.8.4;
 
 import "./L2ArbitrumGateway.sol";
-import {L1USDCGateway} from "../../ethereum/gateway/L1USDCGateway.sol";
+import {L1USDCGateway, IFiatToken} from "../../ethereum/gateway/L1USDCGateway.sol";
 import "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";
 
 /**
@@ -199,8 +199,3 @@ contract L2USDCGateway is L2ArbitrumGateway {
         return true;
     }
 }
-
-interface IFiatToken {
-    function burn(uint256 _amount) external;
-    function mint(address _to, uint256 _amount) external;
-}

contracts/tokenbridge/ethereum/gateway/L1USDCGateway.sol

@@ -9,6 +9,7 @@ import {
     TokenGateway,
     IERC20
 } from "./L1ArbitrumExtendedGateway.sol";
+import {IFiatToken} from "../../libraries/IFiatToken.sol";
 
 /**
  * @title Custom gateway for USDC implementing Bridged USDC Standard.
@@ -84,8 +85,8 @@ contract L1USDCGateway is L1ArbitrumExtendedGateway {
 
     /**
      * @notice Pauses deposits. This can only be called by the owner.
-     * @dev    Pausing is prerequisite for burning escrowed USDC tokens. Incoming withdrawals are not affected.
-     *         It is onetime permanent action. Pausing the withdrawals needs to be done separately on the child chain.
+     * @dev    Pausing is permanent and can't be undone. Pausing is prerequisite for burning escrowed USDC tokens.
+     *         Incoming withdrawals are not affected. Pausing the withdrawals needs to be done separately on the child chain.
      */
     function pauseDeposits() external onlyOwner {
         if (depositsPaused) {
@@ -117,7 +118,7 @@ contract L1USDCGateway is L1ArbitrumExtendedGateway {
             revert L1USDCGateway_L2SupplyNotSet();
         }
 
-        Burnable(l1USDC).burn(_amountToBurn);
+        IFiatToken(l1USDC).burn(_amountToBurn);
         emit GatewayUsdcBurned(_amountToBurn);
     }
 
@@ -175,10 +176,3 @@ contract L1USDCGateway is L1ArbitrumExtendedGateway {
         return l2USDC;
     }
 }
-
-interface Burnable {
-    /**
-     * @notice Circle's referent USDC implementation exposes burn function of this signature.
-     */
-    function burn(uint256 _amount) external;
-}

contracts/tokenbridge/libraries/IFiatToken.sol

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: MIT
+// solhint-disable-next-line compiler-version
+pragma solidity >0.6.0 <0.9.0;
+
+/**
+ * @title IFiatToken
+ * @dev Part of the interface that is used in Circle's referent implementation of the USDC
+ *      Ref: https://github.com/circlefin/stablecoin-evm
+ *
+ *      This interface is used in the L1USDCGateway, L1OrbitUSDCGateway and L2USDCGateway contracts.
+ */
+interface IFiatToken {
+    function burn(uint256 _amount) external;
+    function mint(address _to, uint256 _amount) external;
+}