feat: make OptimismMintableUpgradableERC20.sol upgradable

packages/contracts-bedrock/src/universal/OptimismMintableUpgradableERC20.sol

@@ -1,26 +1,33 @@
 // SPDX-License-Identifier: MIT
 pragma solidity 0.8.15;
 
-import { ERC20 } from "@openzeppelin/contracts/token/ERC20/ERC20.sol";
+import { ERC20Upgradeable } from "@openzeppelin/contracts-upgradeable/token/ERC20/ERC20Upgradeable.sol";
 import { IERC165 } from "@openzeppelin/contracts/utils/introspection/IERC165.sol";
+import { Initializable } from "@openzeppelin/contracts-upgradeable/proxy/utils/Initializable.sol";
 import { ILegacyMintableERC20, IOptimismMintableERC20 } from "src/universal/IOptimismMintableERC20.sol";
 import { ISemver } from "src/universal/ISemver.sol";
 
-/// @title OptimismMintableERC20
-/// @notice OptimismMintableERC20 is a standard extension of the base ERC20 token contract designed
+/// @title OptimismMintableUpgradableERC20
+/// @notice OptimismMintableUpgradableERC20 is a standard extension of the base ERC20 token contract designed
 ///         to allow the StandardBridge contracts to mint and burn tokens. This makes it possible to
 ///         use an OptimismMintablERC20 as the L2 representation of an L1 token, or vice-versa.
 ///         Designed to be backwards compatible with the older StandardL2ERC20 token which was only
 ///         meant for use on L2.
-contract OptimismMintableERC20 is IOptimismMintableERC20, ILegacyMintableERC20, ERC20, ISemver {
+contract OptimismMintableUpgradableERC20 is Initializable, IOptimismMintableERC20, ILegacyMintableERC20, ERC20Upgradeable, ISemver {
     /// @notice Address of the corresponding version of this token on the remote chain.
-    address public immutable REMOTE_TOKEN;
+    /// @notice Breaking the naming convention to remain as close as possible to the non-
+    ///         upgradable version (where this is immutable)
+    address public REMOTE_TOKEN;
 
     /// @notice Address of the StandardBridge on this network.
-    address public immutable BRIDGE;
+    /// @notice Breaking the naming convention to remain as close as possible to the non-
+    ///         upgradable version (where this is immutable)
+    address public BRIDGE;
 
     /// @notice Decimals of the token
-    uint8 private immutable DECIMALS;
+    /// @notice Breaking the naming convention to remain as close as possible to the non-
+    ///         upgradable version (where this is immutable)
+    uint8 private DECIMALS;
 
     /// @notice Emitted whenever tokens are minted for an account.
     /// @param account Address of the account tokens are being minted for.
@@ -42,19 +49,23 @@ contract OptimismMintableERC20 is IOptimismMintableERC20, ILegacyMintableERC20,
     /// @custom:semver 1.3.0
     string public constant version = "1.3.0";
 
+    constructor() {
+        _disableInitializers();
+    }
+
     /// @param _bridge      Address of the L2 standard bridge.
     /// @param _remoteToken Address of the corresponding L1 token.
     /// @param _name        ERC20 name.
     /// @param _symbol      ERC20 symbol.
-    constructor(
+    function initialize(
         address _bridge,
         address _remoteToken,
         string memory _name,
         string memory _symbol,
         uint8 _decimals
-    )
-        ERC20(_name, _symbol)
+    ) initializer public
     {
+        __ERC20_init(_name, _symbol);
         REMOTE_TOKEN = _remoteToken;
         BRIDGE = _bridge;
         DECIMALS = _decimals;