Rethinking the default class hierarchy: Milestone 1, Week 4

[Robert Aron]

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

Hello!

Last week I managed to do the following tasks for #SAOC2021:
  - fixed all the tests(in dmd and druntime) that were failing 
after I introduced `ProtoObject` as the root of all objects
  - modified the DIP according to the reviews from my mentors
  - created a [pr](https://github.com/dlang/DIPs/pull/219) with 
the DIP to the [DIPs repo](https://github.com/dlang/DIPs)
  - implemented a first version of interfaces and mixins for 
`opCmp` and `opEquals`
  - added unittests for `opCmp` and `opEquals`
  - managed to remove `__monitor` from `ProtoObject`

The plan for the next milestone:
  - implement interfaces and mixins for `toHash` and `toString`
  - improve those already implemented
  - add an interface that adds `__monitor` in classes that extends 
`ProtoObject`