Jparray primitive improvements

Final pull request.
The trick is to avoid this HostRef logic. Since Python C API function PyList_SET_ITEM properly handles the refcounts of elements, we avoid manually handling them.
Added testcases for each primitive datatype. JPype does not map JShort for some reason. But this outside of the scope of this PR.
According to some measurements on my laptop the sliced access for 10^7 integers reduces from 2.5s to 200ms, which is a factor of 10!