Subsubtopic: Box<Trait>

[DrizztVD1]

Continuing the discussion from Difficulty of learning rust and further work on the learning curve:

Rust beginner notes & questions

Small off-topic: why it was decided to join Box<SizedType>, Box<[T]> and Box<Trait> under the single Box roof instead of using three separate types for each use-case, e.g. something like Dyn<Trait>for trait objects?