Implement j.u.Hashtable on top of j.u.HashMap.
Implement j.u.Hashtable on top of j.u.HashMap.

Instead of Scala's `HashMap`, which simplifies it a lot.
`Hashtable` is a legacy API that was effectively replaced by
`HashMap` even in Java.

scala-js/scala-js,sjrd/scala-js,sjrd/scala-js,scala-js/scala-js,gzm0/scala-js,scala-js/scala-js,gzm0/scala-js,sjrd/scala-js