Cleanup module declaration and imports

[joulei]

What it does

Defines a uniform way to import modules, switches those messy use super::super::super::super... into always going to the root:

use crate::ecs::{
    systems::{...} 
}

Delegates the submodule declaration to each module, we used to have all modules declared in main.rs

I took a very opinionated decision here, so let me know what you think, we could declare the modules and submodules for our current codebase as follows:

Option A

     - systems // folder 
       - render_system.rs  // contains main render_system module, and sub-module declarations (`pub mod camera`)
       - physics_system.rs // same as above
       - render_sytem  // folder
           - camera.rs
        - physics_system // folder 
           - physics.rs
        ... etc.

Option B

    - system
       - render_sytem  // folder
           - mod.rs // contains main render_system module, and sub-module declarations (`pub mod camera`)
           - camera.rs
        - physics_system // folder 
           - mod.rs
           - physics.rs
        ... etc.

I went for option B, I believe having everything in the same folder makes it clearer and avoids a bunch of scrolling and clicking when navigating graphically through the codebase. It also makes module declaration feel more explicit even if the file also contains code. i.e.: you know you'll always find the mod declaration in the mod.rs file

I'd like to also set up a convention when organizing the imports at the top of the file, for the time being, I used:

1. imports from our own crate
2. imports from std
3. imports from ggez

Unrelated, but also finally ran cargo fmt and removed /docs from the .gitignore file (so you'll see diagrams added)

[mcass19]

I'm from the Option A school, but always prefer this against nothing. Great work ❤️

Could be beneficial to install something like https://github.com/regexident/cargo-modules?