Rethinking the default class hierarchy: Milestone 1, Week 3

[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:
  - finished the to write the DIP
  - analized the code Edi wrote for his talk
  - introduce `ProtoObject` (in compiler and runtime) as the root 
of all objects
  - fixed a part of the tests that were failing

The plan for the next week:
  - modify the DIP(if needed) according to the reviews from my 
mentors
  - continue to fix the tests
  - start to work on interfaces and mixins implementation and 
unittests