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

drop redundant type checks in PyEval_CallObjectWithKeywords() in favour of the simpler and faster PyObject_Call()

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extra : transplant_source : %05%05%B3.Ly1%1C%E8%ED%D8%C2m%2B%7F%C7%05%08z%04

This commit seems to break the py3k build.

This reverts commit b50c6792.

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extra : transplant_source : %DF%9C%95%AE%9B7m%05%AA%02%C9%9C%80L%88O0%CD%40c

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extra : transplant_source : %7B%FB%25%5D%D5Z%84%FD%12%CBC%5B%BD%97%D6%93%FCP%E0%A1

optimise dict.setdefault() - would be better in Builtin.py, but class scopes don't seem to support signature overloading yet

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extra : rebase_source : d0f2bd97e0cec8919ac00e9a0ec8b714e338a9cb