Handle a scope being passed into a scope with add()

[Sinjhin]

Similar to this branch https://github.com/canjs/can-view-scope/tree/scope-in-scope-warning

But instead of unwrapping the scope to get its _context we want to add the functionality to the scope for it to be handled correctly and properly look up values in its context in the scope chain.

[phillipskevin]

To do this, we would want to add the getKeyValue and onKeyValue (and maybe other) symbols to scopes.