Get rid of helpers apply/new JS methods with varargs.

For applying a method with varargs, the codegen directly
emits an appropriate translation.

To instantiate an object with a constructor taking varargs,
instead of a helper, the codegen calls a method of the
scala.scalajs.runtime package object.
Get rid of the helpers {un,}wrapJavaScriptException.

They are replaced by proper Scala.js method in the
scala.scalajs.runtime package object.
js.Dynamic.newInstance returns Object with Dynamic.

Instead of just Dynamic.
Mark js.WrappedDict as inlineable, and override its keys method.
Get rid of the objectClone helper.

By implementing it in Scala.js in the scala.scalajs.runtime
package object.
Merge pull request #1173 from sjrd/get-rid-of-helpers

Get rid of helpers, the easy ones