Add an object literal construction mechanism.

Literal JavaScript objects like
  {foo: 3, bar: "foobar"}
can now be constructed with the syntax
  js.Dynamic.literal(foo = 3, bar = "foobar")
Of course, the `literal` method can be imported in scope with
a short name, e.g.,
  import js.Dynamic.{ literal => lit }
  lit(foo = 3, bar = "foobar")