SCM_I_IS_VECTOR only true for tc7_vector, not weak vectors

SCM_I_IS_VECTOR only true for tc7_vector, not weak vectors

* libguile/tags.h (SCM_TYP7S, SCM_HAS_TYP7S): Remove these, as we no
  longer do the differs-by-one-bit thing for vectors and weak vectors.
* libguile/vectors.h (SCM_I_IS_VECTOR): Use SCM_HAS_TYP7.
  (SCM_I_IS_NONWEAK_VECTOR): Remove.
* libguile/vm-engine.c (vector-length, vector-ref, vector-set!)
  (vector-ref/immediate, vector-set!/immediate): We can inline these
  instructions completely now.

* libguile/vm.c (vm_error_not_a_vector, vm_error_out_of_range): New
  error conditions.