set flag useNewPing.

static <A> A memLimit(long allocationLimit, IF0<A> f) {
  if (f == null) null;
  long baseline = bytesAllocatedByCurrentThread();
  long limit = baseline+allocationLimit;
  temp tempNewPing_addToStack(r {
    if (bytesAllocatedByCurrentThread() > limit)
      fail("Memory limit exceeded: " + toM(allocationLimit));
  });
  A a = f.get();
  ping();
  ret a;
}