1.0.29.6: work around stack-allocated value cell badness in HANDLER-CASE

1.0.29.6: work around stack-allocated value cell badness in HANDLER-CASE

* Use an explicit CONS so the closed-over variable is read-only and
  doesn't need a value cell, and stack allocate the CONS instead.
  (Stack analysis still can't reason about stack allocated
  value-cells... it might be that doing a transformation like this in
  the compiler would be the way to integrate dx value cells properly
  into Python, maybe?)