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.json

@@ -61,6 +61,7 @@
     "ethereumjs-util": "^6.2.0",
     "ganache-core-coverage": "https://github.com/OpenZeppelin/ganache-core-coverage/releases/download/2.5.3-coverage/ganache-core-coverage-2.5.3.tgz",
     "lodash.startcase": "^4.4.0",
+    "lodash.zip": "^4.2.0",
     "micromatch": "^4.0.2",
     "mocha": "^7.1.1",
     "solhint": "^3.0.0-rc.6",