Class/Structs and Box<T>

[arekbal]

SList<T> (for example) is a struct (opposite to class List<T>).
It leads to specific changes in how it behaves. 'Assign' conceptually means 'move' for this structs. Memberwise copy could harm developer.
So, ideally these should be passed by ref.
If user wants it to behave like a plain-old object, it has to wrapped in Box.

Consider making assignment [Obsolete] and enable operations MoveOut (negative length and checks everywhere?), MoveIn ctor.

It would be nice to document this stuff in example and docs.