Fixed the Set.xor function and it's unit tests.
Moved 'mask' and 'matchPrefix' into the endian-specific modules, since they each need a different version of 'mask'.
Remove the BitOps tests, they're not needed anymore.
Fixed the issue with IntSet and IntMap -- some of the functions have a condition which needs to be reversed for big-endian tries. Big-endian is now the default, but little-endian tries can be enabled if the project is compiled with the LITTLE_ENDIAN_TRIES symbol defined.