exercises(high-scores): implement

[ee7]

To-do:

• [x] example, stub: consider renaming parameter from s
• [ ] config.json: decide on exercise placement
• [ ] example, stub, tests: personalTopThree: consider passing an allocator instead.
• [ ] consider making functions return ?u32
• [ ] consider removing test for init
• [ ] consider removing tests for immutability
• [ ] consider making the scores field an ArrayList, so we can better imagine adding new scores.

[ee7]

I don't love this exercise as-is. Maybe it'd be better if it was more flexible. See the to-do items for things I'm considering.

[booniepepper]

Personally I think it's reasonable as-is. We can always riff on it in the future if people have feedback