Core Lib Documentation: `Debug` module

[reviewable-StarkWare]

This change is 

[TAdev0]

@orizi PrintTrait is pub(crate) as well as PrintTrait implementations. We cannot use print method directly anymore, thats why we removed this example

[TAdev0]

@orizi indeed, we can use it with priori editions, my bad. I restored and slightly updated the comment, moving it over the PrintTrait .