@0x/asset-proxy: Add more functions to IDydx.

`@0x/dev-utils`: Fix all the weird dydx base unit madness.

contracts/asset-proxy/contracts/src/interfaces/IDydx.sol

@@ -189,4 +189,13 @@ interface IDydx {
         external
         view
         returns (Value memory supplyValue, Value memory borrowValue);
+
+    // @dev Approves/disapproves any number of operators. An operator is an external address that has the
+    //      same permissions to manipulate an account as the owner of the account. Operators are simply
+    //      addresses and therefore may either be externally-owned Ethereum accounts OR smart contracts.
+    //      Operators are also able to act as AutoTrader contracts on behalf of the account owner if the
+    //      operator is a smart contract and implements the IAutoTrader interface.
+    // @param args A list of OperatorArgs which have an address and a boolean. The boolean value
+    //        denotes whether to approve (true) or revoke approval (false) for that address.
+    function setOperators(OperatorArg[] calldata args) external;
 }

contracts/asset-proxy/contracts/test/TestDydxBridge.sol

@@ -226,6 +226,9 @@ contract TestDydxBridge is
         returns (Value memory supplyValue, Value memory borrowValue)
     {}
 
+    /// @dev Unused.
+    function setOperators(OperatorArg[] calldata args) external {}
+
     /// @dev overrides `_getDydxAddress()` from `DeploymentConstants` to return this address.
     function _getDydxAddress()
         internal