React.Children support for Call and Return types

[MatteoVH]

Include React Calls and Returns (as defined in this sub-repo) in the children enumerated over by React.Children. Should be similar to the Portal support added in #11378. This issue is a follow up to #11373 and was suggested by @gaearon.

[MatteoVH]

I'd like to take this. Should be able to have a PR up tonight. 🙂

[gaearon]

Fixed by #11422.