Rethinking the default class hierarchy: Milestone 2, Weeks 2 & 3

[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 2 weeks I managed to do the following tasks for #SAOC2021:
  - asked the community for feedback(for DIP)
  - continued to reorganize the code
  - continue the implementation (add SynchronizedProtoObject class 
and mixin ImplementOrdered)
  - discuss the feedback with my mentors

The plan for the next week:
  - answer DIP comments
  - discuss with the mentors if the approach for interfaces and 
mixins should be changed
  - continue the implementation (with or without a new approach)
  - make the first PRs

! I decided not to make the PRs yet because there was a lot of 
feedback from the community and I wanted take all of that into 
consideration !