Remove unsafe childFunction

[sgrishchenko]

Part of #358
cc @turansky @aerialist7

childFunction and RProps.children(value: T) signatures are removed because of unsafety.
These methods can lead to inconsistency and mistakes in codebase of kotlin-wrappers users.

Recommended solution for such cases:

interface RenderChildrenProps : RProps {
    var children: RBuilder.(value: Int) -> Unit
}

val RenderChildren = fc<RenderChildrenProps> { props ->
    div {
        props.children(this, 42)
    }
}

val Container = fc<RProps> {
    RenderChildren {
        attrs {
            children = { value ->
                span {
                    attrs {
                        jsStyle {
                            color = "red"
                        }
                    }

                    +"$value"
                }
            }
        }
    }
}

[Leonya]

Released in pre.223