Add ScriptableSeq and ScriptableMap.

- ScriptableSeq is a wrapper for scala.collection.Seq instances that allows
developers to interact with them like it was a native JavaScript array.
- ScriptableMap is a wrapper for scala.collection.Map instances that allows
developers to interact with them like it was a native JavaScript object.

No put and delete methods of JavaNativeObject are overridden so that only
immutables collections are supported.
Make ServerToHandle trait sealed.

When a trait is "sealed" all of its subclasses are declared within the same
file and that makes the set of subclasses finite which allows certain compiler
checks.
Remove unused emptyRoutingTable.
Fix a typo: def -> val.