EnumerableSet improvements (#2077)

* Remove newAddressSet

* Add count and get functions.

* Fix lint

(cherry picked from commit 7988c044e0)

contracts/utils/EnumerableSet.sol

@@ -29,17 +29,6 @@ library EnumerableSet {
         address[] values;
     }
 
-    /**
-     * @dev Creates a new empty address set.
-     */
-    function newAddressSet()
-        internal
-        pure
-        returns (AddressSet memory)
-    {
-        return AddressSet({values: new address[](0)});
-    }
-
     /**
      * @dev Add a value to a set. O(1).
      * Returns false if the value was already in the set.
@@ -105,6 +94,9 @@ library EnumerableSet {
      * @dev Returns an array with all values in the set. O(N).
      * Note that there are no guarantees on the ordering of values inside the
      * array, and it may change when more values are added or removed.
+
+     * WARNING: This function may run out of gas on large sets: use {length} and
+     * {get} instead in these cases.
      */
     function enumerate(AddressSet storage set)
         internal
@@ -117,4 +109,33 @@ library EnumerableSet {
         }
         return output;
     }
+
+    /**
+     * @dev Returns the number of elements on the set. O(1).
+     * Note that there are no guarantees on the ordering of values inside the
+     * array, and it may change when more values are added or removed.
+     */
+    function length(AddressSet storage set)
+        internal
+        view
+        returns (uint256)
+    {
+        return set.values.length;
+    }
+
+   /** @dev Returns the element stored at position `index` in the set. O(1).
+    * Note that there are no guarantees on the ordering of values inside the
+    * array, and it may change when more values are added or removed.
+    *
+    * Requirements:
+     *
+     * - `index` must be strictly less than {length}.
+    */
+    function get(AddressSet storage set, uint256 index)
+        internal
+        view
+        returns (address)
+    {
+        return set.values[index];
+    }
 }