Variant/Value: unsigned type support

Currently we only support signed integer types, because Java traditionally only supported unsigned data types. This changed with Java 8 which added support for unsigned interpretation of data represented by int/long. See [Java Datatypes][https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html). In Java Variant/Value bindings we only allow getting the signed types because we call get() with a signed argument and if the underlying type is unsigned that will fail in the datatype check. I wonder if it not best to augment Variant to better support the bindings by adding a function that allows copying the raw data to a buffer of specified size or something.

Start at the Java Variant/Value binding calls that use get() and compare them with the underlying Variant datatype checks. Define completion as a supported way to retrieve unsigned int and long representations without signed-type rejection, with coverage for the Java 8 unsigned cases described in the issue.