Add EnumerableMap, refactor ERC721 (#2160)

* Implement AddressSet in terms of a generic Set

* Add Uint256Set

* Add EnumerableMap

* Fix wording on EnumerableSet docs and tests

* Refactor ERC721 using EnumerableSet and EnumerableMap

* Fix tests

* Fix linter error

* Gas optimization for EnumerableMap

* Gas optimization for EnumerableSet

* Remove often not-taken if from Enumerable data structures

* Fix failing test

* Gas optimization for EnumerableMap

* Fix linter errors

* Add comment for clarification

* Improve test naming

* Rename EnumerableMap.add to set

* Add overload for EnumerableMap.get with custom error message

* Improve Enumerable docs

* Rename Uint256Set to UintSet

* Add changelog entry

contracts/mocks/EnumerableMapMock.sol

@@ -0,0 +1,38 @@
+pragma solidity ^0.6.0;
+
+import "../utils/EnumerableMap.sol";
+
+contract EnumerableMapMock {
+    using EnumerableMap for EnumerableMap.UintToAddressMap;
+
+    event OperationResult(bool result);
+
+    EnumerableMap.UintToAddressMap private _map;
+
+    function contains(uint256 key) public view returns (bool) {
+        return _map.contains(key);
+    }
+
+    function set(uint256 key, address value) public {
+        bool result = _map.set(key, value);
+        emit OperationResult(result);
+    }
+
+    function remove(uint256 key) public {
+        bool result = _map.remove(key);
+        emit OperationResult(result);
+    }
+
+    function length() public view returns (uint256) {
+        return _map.length();
+    }
+
+    function at(uint256 index) public view returns (uint256 key, address value) {
+        return _map.at(index);
+    }
+
+
+    function get(uint256 key) public view returns (address) {
+        return _map.get(key);
+    }
+}