Children group elements launched before the group itself

[r3v2d0g]

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Hey :wave:!

Currently, to allow ChildrenRef to both contain a list of a ChildRefs and be returned by Bastion::children and Supervisor::children_ref, the children's elements are launched before it (the Childs are launched by the creating method while the Children is launched by the Supervisor that received the Message::Deploy containing it).

We could either:

1. Continue doing that.
2. Remove the elems method from ChildrenRef or replace it to return the number of elements instead of a list of references.
3. Make Bastion::children and Supervisor::children_ref launch the Children themselves and send an handle to the Supervisor instead of the children group.

[onsails]

In option 2 I'd return a list of RefAddrs which is helpful with both identifying and talking to every Child this Children contains. #139 will show decoupling Sender, Receiver and a Child could work.