(VECTOR_MARK, VECTOR_UNMARK, VECTOR_MARKED_P): New macros.

(VECTOR_MARK, VECTOR_UNMARK, VECTOR_MARKED_P): New macros.
(GC_STRING_BYTES): Don't mask markbit (it's only used on `size').
(allocate_buffer): Move.
(string_bytes): Don't mask markbit of `size_byte'.
(mark_maybe_object, mark_maybe_pointer, Fgarbage_collect)
(mark_object, mark_buffer, survives_gc_p, gc_sweep):
Use the `size' field of buffers (rather than the `name' field) for
the mark bit, as is done for all other vectorlike objects.
Use the new macros to access the mark bit of vectorlike objects.