Fix C stubs to avoid returning void

[ncik-roberts]

Fix C stubs to return Val_unit instead of void. Prior to this PR, code like this would segfault:

utop # let t = Bigstringaf.create 100;;
utop # let q = Queue.create ();;
utop # Queue.add (Bigstringaf.blit_to_bytes t ~src_off:50 (Bytes.create 100) ~dst_off:50 ~len:50) q;;
- : unit = ()
utop # Gc.full_major ();;
Segmentation fault (core dumped)