jruntime: Towards compartmentalized accessor class

Biggest hurdle was passing through an instance method to JS. It's done
in a bit of a roundabout way, but very necessary, we're working around
a few nashorn bugs, namely:

https://bugs.openjdk.java.net/browse/JDK-8056978

And the fact that exported java functions don't support the JS `apply`
method.
jruntime: actually implement get_parameter
jruntime: implement get, set; enforce ouput types