add flag to vtables to indicate that their layout is valid

add flag to vtables to indicate that their layout is valid

* libguile/struct.h (SCM_VTABLE_FLAG_VALIDATED): New flag, indicates
  that the layout of a vtable has been validated. The other flags have
  been renumbered.

* libguile/struct.c (scm_i_struct_inherit_vtable_magic): Set the
  VALIDATED flag if everything goes through.
  (scm_struct_vtable_p): If the struct should be a vtable but isn't
  validated, throw an error.
  (scm_make_vtable_vtable): Validate the incoming user_fields layout
  bit. Set the VALIDATED flag.
  (scm_c_make_structv): Add a comment about the case in which we delay
  scm_i_struct_inherit_vtable_magic.