VM tweaks

* libguile/vm-engine.c (VM_CHECK_OBJECT, VM_CHECK_FREE_VARIABLES): Set
  to 0 for both engines.  These are really internal debugging variables,
  which don't affect user-visible features, provided that the compiler
  is correct of course.
  (VM_CHECK_UNDERFLOW): New var, also off by default: whether to check
  for stack underflow when popping values.
  (vm_engine): Don't declare object_count if we are not checking object
  table accesses.

* libguile/vm-engine.h (CACHE_PROGRAM): Don't muck with object_count
  if we are not checking object table accesses.
  (CHECK_UNDERFLOW, PRE_CHECK_UNDERFLOW): Nop out if we are not checking
  underflow.
  (POP2, POP3): New macros which check for underflow before popping more
  than one value.

* libguile/vm-i-loader.c (load_array):
* libguile/vm-i-scheme.c (set_car, set_cdr, vector_set, slot_set)
  (BV_SET_WITH_ENDIANNESS, BV_FIXABLE_INT_SET, BV_INT_SET)
  (BV_FLOAT_SET):
* libguile/vm-i-system.c (partial_cont_call, fix_closure, prompt)
  (fluid_set): Use POP2 / POP3.
  (local_set, long_local_set): Pop to locals instead of using values on
  the stack then dropping; allows for underflow to be checked before the
  value is accessed.
  (BR): Don't NULLSTACK or DROP after the operation.
  (br_if, br_if_not, br_if_eq, br_if_not_eq, br_if_null)
  (br_if_not_null): Pop to locals before doing the compare and jump.

libguile/vm-i-loader.c

@@ -92,8 +92,7 @@ VM_DEFINE_LOADER (106, load_array, "load-array")
   SCM type, shape;
   size_t len;
   FETCH_LENGTH (len);
-  POP (shape);
-  POP (type);
+  POP2 (shape, type);
   SYNC_REGISTER ();
   PUSH (scm_from_contiguous_typed_array (type, shape, ip, len));
   ip += len;