Move exercises to subdirectory

[kytrinyx]

The problems api (x-api) now supports having exercises collected in a subdirectory
named exercises.

That is to say that instead of having a mix of bin, docs, and individual exercises,
we can have bin, docs, and exercises in the root of the repository, and all
the exercises collected in a subdirectory.

In other words, instead of this:

x{TRACK_ID}/
├── LICENSE
├── README.md
├── bin
│   └── fetch-configlet
├── bowling
│   ├── bowling_test.ext
│   └── example.ext
├── clock
│   ├── clock_test.ext
│   └── example.ext
├── config.json
└── docs
│   ├── ABOUT.md
│   └── img
... etc

we can have something like this:

x{TRACK_ID}/
├── LICENSE
├── README.md
├── bin
│   └── fetch-configlet
├── config.json
├── docs
│   ├── ABOUT.md
│   └── img
├── exercises
│   ├── bowling
│   │   ├── bowling_test.ext
│   │   └── example.ext
│   └── clock
│       ├── clock_test.ext
│       └── example.ext
... etc

This has already been deployed to production, so it's safe to make this change whenever you have time.