This reverts commit 21897347.

Turns out CPython broke on us at the same time this change was made.

--HG--
extra : rebase_source : 86b0a49e72ad7f05aa7647350e2116475ab13589

another partial rewrite of the with statement: let the WithTargetAssignmentStatNode do the __enter__() result cleanup explicitly

  (not optimising the generic case here as it has a couple of tricky error cases)
* replaced DECREF(PyList_GET_ITEM()) by separate GET+SET+DECREF calls to prevent
  inconsistent list states during DECREF()

 T731 locals() scope, eval(), exec()

also optimise left-side multiplication of lists/tuples for integer factors, generally fix constant folding test