exercises(high-scores): implement

[ee7]

To-do:

• 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