Issue #123: Free stack elements when they are popped.
Issue #123: Free stack elements when they are popped.

This is quick & dirty, see all the TODOs in this commit.

SWP-Ubau-SoSe2014-Haskell/SWPSoSe14,SWP-Ubau-SoSe2014-Haskell/SWPSoSe14