feat(contracts): Add an address getter to the erc20 bridge interfaces

packages/contracts/contracts/optimistic-ethereum/OVM/bridge/tokens/OVM_L1StandardBridge.sol

@@ -33,7 +33,7 @@ contract OVM_L1StandardBridge is iOVM_L1StandardBridge, OVM_CrossDomainEnabled {
      * External Contract References *
      ********************************/
 
-    address public l2TokenBridge;
+    address public override l2TokenBridge;
 
     // Maps L1 token to L2 token to balance of the L1 token deposited
     mapping(address => mapping (address => uint256)) public deposits;

packages/contracts/contracts/optimistic-ethereum/OVM/bridge/tokens/OVM_L2StandardBridge.sol

@@ -33,7 +33,7 @@ contract OVM_L2StandardBridge is iOVM_L2ERC20Bridge, OVM_CrossDomainEnabled {
      * External Contract References *
      ********************************/
 
-    address public l1TokenBridge;
+    address public override l1TokenBridge;
 
     /***************
      * Constructor *

packages/contracts/contracts/optimistic-ethereum/iOVM/bridge/tokens/iOVM_L1ERC20Bridge.sol

@@ -33,6 +33,12 @@ interface iOVM_L1ERC20Bridge {
      * Public Functions *
      ********************/
 
+    /**
+     * @dev get the address of the corresponding L2 bridge contract.
+     * @return Address of the corresponding L2 bridge contract.
+     */
+    function l2TokenBridge() external returns(address);
+
     /**
      * @dev deposit an amount of the ERC20 to the caller's balance on L2.
      * @param _l1Token Address of the L1 ERC20 we are depositing

packages/contracts/contracts/optimistic-ethereum/iOVM/bridge/tokens/iOVM_L2ERC20Bridge.sol

@@ -43,6 +43,12 @@ interface iOVM_L2ERC20Bridge {
      * Public Functions *
      ********************/
 
+    /**
+     * @dev get the address of the corresponding L1 bridge contract.
+     * @return Address of the corresponding L1 bridge contract.
+     */
+    function l1TokenBridge() external returns(address);
+
     /**
      * @dev initiate a withdraw of some tokens to the caller's account on L1
      * @param _l2Token Address of L2 token where withdrawal was initiated.