(scm_t_cell): Fields are now of type SCM instead of

(scm_t_cell): Fields are now of type SCM instead of
scm_t_bits.  Updated all users.
(SCM_GC_CARD_SIZE_MASK): Use SCM_GC_SIZEOF_CARD instead of
duplicating the code.
(SCM_CELL_OBJECT_LOC): New.
(SCM_CARLOC, SCM_CDRLOC): Use it instead of SCM_CELL_WORD_LOC.
(SCM_CELL_WORD_LOC): Moved to "deprecated.h".