Rethinking the default class hierarchy: Milestone 2, Week 1

[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:

- started to reorganize the code
- implemented interfaces and mixins for `toHash`
- decided the order of PRs I must make next week (first I will 
add the `ProtoObject` class in druntime and after that I will add 
the code that makes `ProtoObject` the root of `Object` in dmd)

The plan for the next week:
- make PRs to [druntime](https://github.com/dlang/druntime) and 
[dmd](https://github.com/dlang/dmd) repos
- continue to reorganize the code
- prepare the next PRs - remove `__monitor` from `ProtoObject` 
and add a new class `SynchronizedProtoObject` (with `__monitor`)