Add NullUndefOr

[oyvindberg]

Add a set of types for working with Javascript types which can be null/undefined.

In particular, we want to use inheritance to enable us to model the following narrowing of optionality in subtypes. This compiles in Typescript under --strictNullChecks:

interface A {
    a: number | null | undefined
}

interface B extends A {
    a: number | null
}

interface C extends A {
    a: number | undefined
}

interface D extends B, C {
    a: number
}

Using the type hierarchy in this PR we can express this in Scala.js like this:

  trait A extends js.Object {
    val a: NullUndefOr[Int] = js.undefined.asInstanceOf[NullUndefOr[Int]] // note 1
  }

  trait B extends A {
    override val a: NullOr[Int]
  }

  trait C extends A {
    override val a: UndefOr[Int] = js.undefined.asInstanceOf[UndefOr[Int]] // note 1
  }

  trait D extends B with C {
    override val a: Required[Int] // note 2
  }

• Note 1: Requires change in scala.js to accept js.undefined.asInstanceOf[X] on rhs in @ScalaJSDefined traits.
• Note 2: This won't work very well when producing Ds, it's impossible to tell scalac that a is not implemented anymore. OTOH consuming this structure coming from javascript is straightforward with a safe d.a.get