improve exercise order (#1912)

I believe some work went into the exercise order by the previous
maintainers, so I didn't completely redo it. I only fixed the obviously
bad aspects of the order:

- difficulty 10 exercises being way too early in the track
- new, easy exercises being at the end of the exercise list

forum discussion:
https://forum.exercism.org/t/rust-track-exercise-order/10855