new transform that converts for-in-dict.iter*() into a while-loop over PyDict_Next(), which makes the loop 30-50% faster

- only supports code in strings, not files (should be easy to add)
- only the "exec XYZ in GLOBALS [,LOCALS]" form is supported, not the execution in the current namespace (would require a mapping representation of it)
- also includes an incomplete (3-args only) and untested implementation for the exec() function in Py3

more balancing needs to be done.