Arities as containers

Proposed by @eparejatobes 
Why the same container?
As I understand, now it's kind of a special composition. Why not to make it a "combinator"? But with explicit container parameter, smth like `MapOver[P <: AnyPath, C <: AnyContainer]` which will return a combination of containers. Then we can have just a method `map` which derives the second parameter for `MapOver`. Also we should have a `Flatten` combinator, which will "multiply" this combination of constructors. Same for `flatMap`.