Jparray primitive improvements

This PR improves the speed of sliced array access by the factor of 10. Its currently only implemented for primitive datatypes, since I needed fast access to them. Works for me and my testcases (which I should really integrate into the provided testsuite). The access pattern is stolen from apache JCC. I've checked with valgrind that no memleaks occur. But please test it before merging it!

Thanks

BTW: why is the testsuite so messed up? Nearly everything if failing with current HEAD. Would love to develop with a working testsuite :)