ZeroEx: Add Unused to StorageId enum

contracts/zero-ex/contracts/src/features/Migrate.sol

@@ -72,7 +72,7 @@ contract Migrate is
             // If the owner is already set to ourselves then we've reentered.
             _rrevert(LibMigrateRichErrors.AlreadyMigratingError());
         }
-        // Temporarily set the owner to ourselves.
+        // Temporarily set the owner to ourselves to enable admin functions.
         ownableStor.owner = address(this);
         stor.migrationOwner = prevOwner;
 

contracts/zero-ex/contracts/src/features/Ownable.sol

@@ -48,7 +48,7 @@ contract Ownable is
     /// @param impl the actual address of this feature contract.
     /// @return success Magic bytes if successful.
     function bootstrap(address impl) external returns (bytes4 success) {
-        // Set the owner.
+        // Set the owner to ourselves to allow bootstrappers to call `extend()`.
         LibOwnableStorage.getStorage().owner = address(this);
 
         // Register feature functions.

contracts/zero-ex/contracts/src/storage/LibStorage.sol

@@ -30,6 +30,7 @@ library LibStorage {
 
     /// @dev Storage IDs for feature storage buckets.
     enum StorageId {
+        Unused, // Unused buffer for state accidents.
         Proxy,
         SimpleFunctionRegistry,
         Ownable,
@@ -44,7 +45,6 @@ library LibStorage {
         pure
         returns (uint256 offset)
     {
-        // We don't use safeMul here to save gas.
         // This should never overflow with a reasonable `STORAGE_OFFSET_MULTIPLIER`
         // because Solidity will do a range check on `storageId` during the cast.
         return uint256(storageId) * STORAGE_OFFSET_MULTIPLIER;