cleanup.

cleanup.

* src/Sbitset.c (Sbitset__new_on_obstack): Use Sbitset instead
of char*.
(Sbitset__isEmpty): Use Sbitset instead of char*.
* src/Sbitset.h (Sbitset): Make it a pointer to unsigned char
instead of char.  This helps to avoid casting errors.
(Sbitset__or): Use Sbitset instead of char*.
(cherry picked from commit 5297ebb3bcdf4f957bdab9c5535c2c9c47f7dc07)