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.