Rethinking the default class hierarchy: Milestone 2, Week 4

[Robert Aron]

[Project 
description](https://gist.github.com/edi33416/0e592f4afbeb2fb81d3abf235b9732ce)
[SAOC 
projects](https://dlang.org/blog/2021/08/30/saoc-2021-projects/)
[DIP](https://github.com/dlang/DIPs/pull/219)

Hello!

Last week I managed to do the following tasks for #SAOC2021:
- answer DIP comments
- made the first PRs
- continued the implementation

The plan for the next milestone:
- provide an implementation that uses templates for `equals`, 
`cmp`, `hash`
- provide a way to make `Ordered`, `Equals` and `Hash` agree with 
each other
- continue to answer comments