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
doc fix for xts
xts doc update for explicit backend
change xts docsetup for tests to be less confusing
remove unneeded test
revert comparison that should be in a separate PR