* Drafted Enumerable.sol.

* Drafted test framework.

* Tweaked the tests to follow oz structure.

* Coded EnumerableSet.

* Moved EnumerableSet to `utils`.

* Fixed linting.

* Improved comments.

* Tweaked contract description.

* Renamed struct to AddressSet.

* Relaxed version pragma to 0.5.0

* Removed events.

* Revert on useless operations.

* Small comment.

* Created AddressSet factory method.

* Failed transactions return false.

* Transactions now return false on failure.

* Remove comments from mock

* Rename mock functions

* Adapt tests to code style, use test-helpers

* Fix bug in remove, improve tests.

* Add changelog entry

* Add entry on Utils doc

* Add optimization for removal of last slot

* Update docs

* Fix headings of utilities documentation

* Simplified mock.

* Fixed comment.

* Revert "Fixed comment."

This reverts commit 39627f9a6f90d48be20ca1449fe03e1699b48173.

* Revert "Simplified mock."

This reverts commit 67468e464b038736251f18e1dc2e368c9091a87a.

* Simplified mock.

Co-authored-by: Nicolás Venturo <nicolas.venturo@gmail.com>
(cherry picked from commit 8975289c)