Reorder Problems to (hopefully) improve learning
Reorder Problems to (hopefully) improve learning

Fixes #127

This commit follows the problem order I suggested in #127, with one
exception. I've put sublist ahead of allergies. Both use enum, but the
enums in allergies can be more of a hassle, as evidenced by all of the
ways I've seen students implement them.

Sublist does have the disadvantage of also requiring a solution that is
generic over multiple types. But I think the best thing to do is keep an
eye on students that reach this part of the track.

Or the later introduction of new exercises could improve this flow.

sacherjj/rust,exercism/xrust,petertseng/exercism-rust,petertseng/exercism-rust,exercism/xrust,sacherjj/rust,petertseng/exercism-rust,sacherjj/rust