Pass child elements in as a third argument to component's body

[fewkz]

Allows you to write the following code

local Button = froact.c({}, function(props, hooks, children)
    return froact.ImageButton({
        BackgroundColor3 = props.color
     }, children)
end)
local element = Button({}, {
    UIScale = froact.UIScale({ Scale = 2 })
})

rather than having to get children via props[froact.Roact.Children]
This also removes that key from props, so you must use children via the third argument from now on.

[fewkz]

We should probably do the same thing for templates, by passing a second arg to the onUpdate callback functions with the children

[fewkz]

turns out this broke everything somehow? I should of noticed earlier...