[v3] Implement new Concept Exercise: `Reduce`

This issue describes how to implement the `Reduce` concept exercise for the Clojure 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 up on the following documents:

- [The features of v3](https://github.com/exercism/v3/blob/master/docs/features-of-v3.md).
- [Rationale for v3](https://github.com/exercism/v3/blob/master/docs/rationale-for-v3.md).
- [What are concept exercise and how they are structured?](https://github.com/exercism/v3/blob/master/docs/concept-exercises.md)

Please also watch the following video:

- [The Anatomy of a Concept Exercise](https://www.youtube.com/watch?v=gkbBqd7hPrA).

## Goal

The goal of this exercise is to teach the student how the `reduce` function is implemented in Clojure.

## Learning objectives

The student will gain a general understanding of:

- The concept of reducing a function on a collection

## Out of scope

- Transducers

## Concepts

- `higher-order-functions`

## Prerequisites

- `basics`

## Resources to refer to

[Clojure for the Brave and True - Core Functions in Depth](https://www.braveclojure.com/core-functions-in-depth/)

## Representer

The [Clojure Representer](https://github.com/exercism/clojure-representer) is currently being implemented and contributions are welcome.

## Analyzer

The [Clojure Analyzer](https://github.com/exercism/clojure-analyzer) is currently being implemented and contributions are welcome.

## Implementing

To implement a concept exercise, the following files must be created:

```
languages
└── clojure
└── exercises
└── concept
└──
├── .docs
| ├── instructions.md
| ├── introduction.md
| ├── hints.md
| └── after.md (optional)
├── .meta
| |── design.md
| └── Example.clj
├── .clj
└── _test.clj
```

## Help

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