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

package-lock.json

@@ -31087,6 +31087,12 @@
         "lodash._reinterpolate": "^3.0.0"
       }
     },
+    "lodash.zip": {
+      "version": "4.2.0",
+      "resolved": "https://registry.npmjs.org/lodash.zip/-/lodash.zip-4.2.0.tgz",
+      "integrity": "sha1-7GZi5IlkCO1KtsVCo5kLcswIACA=",
+      "dev": true
+    },
     "log-symbols": {
       "version": "3.0.0",
       "resolved": "https://registry.npmjs.org/log-symbols/-/log-symbols-3.0.0.tgz",