Reorganizing the code tree.

I'd love to do some cleanup in the code tree. I find it very confusing, even after almost 3 years working in it.

Right now, this issue is up for discussion, we are not expecting anyone to do anything until there is a consensus. The first question is whether you agree that it's, on principle, something worth doing, whether you oppose it, or whether you don't care (I'm not asking whether you want to work on it yourself, I'm fine doing it myself, or getting help of whoever people also belive it'll improve the state of the codebase and contributors work)

If we agree that it's worth doing a change, I suggest a new organization of the tree in https://docs.google.com/document/d/1IfEdbSWqWdpWWZUkFiENqty7twAupyJou7JakDvHkEo/edit?usp=sharing
I believe a design document is better than an issue because anyone can comment any specific point, and we can track discussions of each directory proposal independently.