(scm_bitvector_elements,

(scm_bitvector_elements,
scm_bitvector_writable_elements): Use a scm_t_array_handle and
deliver offset, length and increment to caller.  Changed all uses.
(scm_bitvector_release_elements,
scm_frame_bitvector_release_elements,
scm_bitvector_release_writable_elements,
scm_frame_bitvector_release_writable_elements): Removed.
(scm_array_handle_bit_elements,
scm_array_handle_bit_writable_elements,
scm_array_handle_bit_elements_offset): New.
(scm_make_typed_array): The special value for non-initialized
arrays is now SCM_UNSPECIFIED.  The old special value SCM_BOOL_F
was a valid value to fill bitvectors with, so it can't really be
specialed out.