Type inference for keyed `children` modifier

[armanbilge]

Really interesting ideas from @kubukoz:

type ElemOfS[A] =
  A match {
    case fs2.Stream[_, a] => a
  }

type ElemOfL[A] =
  A match {
    case List[a] => a
  }

children[ElemOfL[ElemOfS[stream.type]]] <-- stream

[kubukoz]

Another idea we discussed, for posterity:

case class Data(id: Int, text: String)

val stream: Stream[IO, List[Data]]

children <-- stream.map {
  _.map(data => data.id -> div(data.text))
}

[armanbilge]

Ah yes, I think I serialized that in https://github.com/armanbilge/calico/issues/85 :)