typo in a comment.
Replace the WT_REF key union's on-page key structure with a uint64_t,
then use shift operations to pull it apart.   This removes the
little-endian requirement from this part of the system and allows us to
rely less on how the compiler lays out structures.  (I'm leaving the
little-endian check I added in place, there are some other things we'd
have to fix to fully support big-endian systems.)

Ref #594.