Improve lookupOrd performance.

This shaves of ``` 100 ns ```  benefiting all functions that use ``` lookupOrd  ```.

I also have a question regarding the use of ``` byteCompare ```. By adding an ``` Ord ``` constraint either on the functions that use ``` byteCompare ``` or by extending the constraint on the ``` CritBitKey ``` typeclass from ``` Eq ``` to ``` Ord ``` we gain another ``` 40 ns ```. I am asking here for what do you think the best route would be ? this would benefit the ``` split ``` functions and those based on ``` lookupOrd ```.

Library was compiled by GHC 7.6.2.