Remove js.prim.* types, and everything related.

The type aliases js.Boolean, js.Number, js.String and
js.Undefined have been removed. The documentation specifies
that the Scala types should be used.

Operations available on JavaScript numbers and strings that do
not have an equivalent in the Scala/Java library can be made
available through imports of implicit pimps of Int/Double and
String:

  import js.JSNumberOps._
  import js.JSStringOps._

Working with dynamically typed JavaScript-style code used to
benefit from js.prim.* types for various things. To restore
(and improve) that kind of code, a few chosen implicit conversions
can be imported:

  import js.DynamicImplicits._

This fixes #1384.
Merge pull request #1390 from sjrd/remove-js-prim-types

Remove js.prim.* types, and everything related.