Implement new Concept Exercise: `macros`

## Learning objectives

- `defmacro`
- the `Macro` module
- what's an AST
- when *not* to use macros

## Out of scope

I am not sure if explaining `require`/`use` in the same exercise would be a good idea. It's a lot of difficult concepts at once. But maybe it makes sense?

## Concepts

- `macros`

## Prerequisites

Come up with something that will put this concept far down the concept tree. Maybe this should depend on a non-existent yet concept of dynamically defining functions without macros? (see https://github.com/exercism/elixir/pull/583#discussion_r567406673)

## Practice exercises

Those practice exercises should have `macros` in their prerequisites and as `practices`:
- `dot-dsl`