Source tree admin - put each sub-class in its own directory?

At the moment, each session lives in its own directory within `src`. As an instructor, I have found this hard to navigate. Most specifically, I've found it hard to find the `concurrency` module when I've been trying to do so on the fly.

```
src
android
bare-metal
borrowing
cargo
...
concurrency

```

I would personally like to see the sessions within the Rust Fundamentals, i.e. the main course, be put into their own directory. We could also create a `welcome` module for the prelude material. Something like this:

```
src
android
bare-metal
chromium
concurrency
fundamentals
borrowing
cargo
...
welcome
...
```