[v3] Implement new Concept Exercise: sets

This issue describes how to implement the sets concept exercise for the Swift track.

## Getting started

**Please please please read the docs before starting.** Posting PRs without reading these docs will be a lot more frustrating for you during the review cycle, and exhaust Exercism's maintainers' time. So, before diving into the implementation, please read the following documents:

- [The features of v3][docs-features-of-v3].
- [Rationale for v3][docs-rationale-for-v3].
- [What are concept exercises and how are they structured?][docs-concept-exercises]

Please also watch the following video:

- [The Anatomy of a Concept Exercise][anatomy-of-a-concept-exercise].

## Goal

This concept exercise should convey a basic understanding of how to work with Sets in Swift.

## Things to teach

- Initializing empty sets
- Initializing sets from array literals
- Inserting and removing elements
- Testing for membership
- Iterating over a set
- Performing set operations

## Things not to teach

- Hashable requirement for elements
- map, reduce, filter, etc.

## Concepts

- `sets`
- `set-operations`

## Prerequisites

- `arrays`

## Resources to refer to

### Hints

- [Sets][sets]
- [Set documentation][set-docs]

### After

- [Sets][sets]
- [Set documentation][set-docs]

## Representer

TBD

## Analyzer

TBD

## Implementing

See [the Swift track implementation guide][swift-implementing-concept-exercises] for guidance.

## Help

If you have any questions while implementing the exercise, please post the questions as comments in this issue.

[swift-implementing-concept-exercises]: https://github.com/wneumann/v3/blob/master/languages/swift/reference/implementing-a-concept-exercise.md
[sets]: https://docs.swift.org/swift-book/LanguageGuide/CollectionTypes.html#ID484
[set-docs]: https://developer.apple.com/documentation/swift/Set
[docs-concept-exercises]: https://github.com/exercism/v3/blob/master/docs/concept-exercises.md
[docs-rationale-for-v3]: https://github.com/exercism/v3/blob/master/docs/rationale-for-v3.md
[docs-features-of-v3]: https://github.com/exercism/v3/blob/master/docs/features-of-v3.md
[anatomy-of-a-concept-exercise]: https://www.youtube.com/watch?v=gkbBqd7hPrA