Switch constant folding behavior for precompilation

[shirok]

Precomputed constants must be serializable if the code is precompiled. There's no restriction in the normal mode. The switch needs to be done inside optimizer. The suggested fix:

(define (pass3/precompute-constnat ...)
   ...
   [(r) (and (or (not precompiling?)
                 (serializable? r))
             ($const r))]
  ...