XTS support
add the actual xts tests
refactor XTS to require split keys

* First half to AES object, second half to XTS
* Add split_key clsasmethod on XTS
* Revert AES object to proper key_sizes
* Add initial docs (improvements welcome)
additional key data should be required, checked for size
expand explanation for split key in docs
reorganize xts tests & add skip support
add xts vector load tests