remove support for "entities" -- a form of applicable struct

Entities were meant to be a form of applicable struct. Unfortunately,
the implementation is intertwingled with generics. Removing them, for
now, will make it possible to cleanly re-add applicable struct support.

* libguile/struct.h (SCM_STRUCTF_ENTITY): Remove.
  (SCM_STRUCTF_GOOPS_HACK): New flag; sigh.
* libguile/struct.c (scm_make_struct): We make "entity" structs if the
  GOOPS_HACK flag is set. This will be fixed when we rework flags and
  remove hidden words.

* libguile/goops.c (scm_class_of): Structs are not applicable, for now
  at least.
  (scm_sys_inherit_magic_x, scm_basic_basic_make_class)
  (scm_sys_allocate_instance, scm_sys_set_object_setter_x):
  (make_struct_class): Adapt for no more entities (and thus no entity
  flag).
  (create_standard_classes): For some reason, generic functions were
  getting the LIGHT flag set, after the ENTITY flag was removed; so for
  now explicitly clear that flag.
* libguile/goops.h (SCM_GENERIC_SETTER, SCM_SET_GENERIC_SETTER): New
  macros.

* libguile/objects.h:
* libguile/objects.c: Remove code for entities.

* libguile/debug.c: (scm_procedure_source): Only work with generics.

* libguile/eval.c (scm_trampoline_0, scm_trampoline_1)
  (scm_trampoline_2): Only handle generics.

* libguile/eval.i.c (CEVAL): #ifdef out the pieces about entities.

* libguile/procprop.c (scm_i_procedure_arity): Remove support for
  entities.

* libguile/procs.c (scm_procedure_p, scm_procedure, scm_setter): Remove
  entity support.

libguile/debug.c

@@ -356,7 +356,7 @@ SCM_DEFINE (scm_procedure_source, "procedure-source", 1, 0, 0,
         }
     }
   case scm_tcs_struct:
-    if (!SCM_I_ENTITYP (proc))
+    if (!(SCM_OBJ_CLASS_FLAGS (proc) & SCM_CLASSF_PURE_GENERIC))
       break;
     goto procprop;
   case scm_tc7_smob: