Fix memoization bug
Fix memoization bug

* src/pf/optimize.lua: Keep objects in the cache alive.  Before, this
  wasn't happening because the objects were being stringified to be made
  as keys to the cache.  This bug could cause spurious cache hits, if a
  cached arg was collected and a new table was allocated at the same
  memory address.

SnabbCo/pflua,mpeterv/pflua