Add .let cases to set immutable key values

[RoyalIcing]

• Skips on rerenders, to make rerendering fasters

Choice 1

enum ButtonProp {
  case let(…)
  case set(…)
}

func button(_ key: String, _ props: [ButtonProp<Msg>]) -> ViewElement<Msg>

Choice 2

func button(_ key: String, let: [ButtonProp<Msg>] = [], _ var: [ButtonProp<Msg>]) -> ViewElement<Msg>

Choice 3

func button(_ key: String, _ props: [ButtonProp<Msg>], letProps: ButtonProp<Msg>…) -> ViewElement<Msg>

Choice 4

struct ViewElementTemplate<Prop> {
  var key: String
  var props: [Prop]
  private var makeElement: (String, [Prop]) -> ViewElement

  init(_ key: String, `let` letProps: [Prop]) {
    self.key = key
    self.props = letProps
  }

  func vary(_ varProps: Prop) {
    var props = self.props
    props.append(varProps)
    return makeElement(key, props)
  }
}

func button(_ key: String, `let` letProps) -> ButtonTemplate<Msg>