rework vm tracing

* libguile/vm-engine.c (VM_NAME): Engines take the VM itself (not the
  vp), so they can pass the VM to hooks. No more hook args, we dispatch
  without them.

* libguile/vm-engine.h (RUN_HOOK): Dispatch the hook if the trace level
  is positive (instead of if the hook is there). Don't cache registers
  on return from the dispatch.

* libguile/vm.h:
* libguile/vm.c (vm_dispatch_hook): Don't bother with a dynwind; instead
  decrement the trace level when going into a hook, and if we have a
  nonlocal exit, the trace level never gets incremented again. Worse is
  better.
  (make_vm, scm_vm_trace_level, scm_set_vm_trace_level_x): New concept,
  trace level. If positive, we run the hooks, otherwise we don't. Should
  work. Removed scm_vm_trace_frame, I don't think that was the right way
  to do it.

* module/system/vm/vm.scm: Replace vm-trace-frame with vm-trace-level
  and set-vm-trace-level!; the hooks actually get the frame as an
  argument now.

libguile/vm-engine.c

@@ -34,12 +34,13 @@
 
 
 static SCM
-VM_NAME (struct scm_vm *vp, SCM program, SCM *argv, int nargs)
+VM_NAME (SCM vm, SCM program, SCM *argv, int nargs)
 {
   /* VM registers */
   register scm_t_uint8 *ip IP_REG;	/* instruction pointer */
   register SCM *sp SP_REG;		/* stack pointer */
   register SCM *fp FP_REG;		/* frame pointer */
+  struct scm_vm *vp = SCM_VM_DATA (vm);
 
   /* Cache variables */
   struct scm_objcode *bp = NULL;	/* program base pointer */
@@ -53,10 +54,6 @@ VM_NAME (struct scm_vm *vp, SCM program, SCM *argv, int nargs)
   int nvalues = 0;
   SCM finish_args;                      /* used both for returns: both in error
                                            and normal situations */
-#if VM_USE_HOOKS
-  SCM hook_args = SCM_EOL;
-#endif
-
 #ifdef HAVE_LABELS_AS_VALUES
   static void **jump_table = NULL;
 #endif