Always create the bytevector SMOB type.

Always create the bytevector SMOB type.

* libguile/bytevectors.c (scm_tc16_bytevector, print_bytevector,
  bytevector_equal_p, free_bytevector): Don't use the snarfing macros.
  (scm_bootstrap_bytevectors): New.
  (scm_init_bytevectors): No longer initialize SCM_NULL_BYTEVECTOR,
  which is done by `scm_bootstrap_bytevectors ()'.

* libguile/bytevectors.h (scm_bootstrap_bytevectors): New declaration.
  (scm_init_bytevectors): Made internal.  This can be done because we
  explicitly register it with `scm_c_register_extension ()' in
  `scm_bootstrap_bytevectors ()'.

* libguile/init.c (scm_i_init_guile): Call `scm_bootstrap_bytevectors ()'.
  This is so that expressions like "(generalized-vector-length #vu8())"
  work even when `(rnrs bytevector)' hasn't been loaded.