smart_pointers and conversions exercises not in alphabetical order.

[Nahor]

Except for smart_pointers and conversions, all the exercises are in alphabetical order, which is convenient since one can then just open them one after the other. With those two though, one has to look at rustlings' output to see which exercise is next. Worse, since one gets used to the order from previous exercises, one may not notice immediately and get confused when the compilation errors within rustlings don't match with the code (e.g. fixing arc1.rs while rustlings compiles box1.rs)

I suggest to rename the exercises like the sections _. This would allow to keep meaningful name for each exercises if desired (arc, try_from_into) while still having an intuitive and explicit order.