Attached is a complete AC conversion of itertools.

This is a large conversion, so I was extra careful. I even went over the entire diff manually to check for any errors. Compilation runs without warnings and the entire test suite passes.

Notes:
* I didn't convert itertools.repeat due to the ongoing discussion regarding it.
* I didn't convert itertools.islice since its signature is like range() but with an additional first required argument, making it impossible to convert properly.
* I didn't convert several __new__ methods because they treat *args similarly to zip(): itertools.chain, itertools.product, itertools.zip_longest
* I used Larry's "nullable ints" patch (see issue20341) for the 'r' parameter to itertools.permutations.
* I converted all of the following methods when defined: __new__ (except for classes mentioned above), __sizeof__, __reduce__, __setstate__, __copy__, __length_hint__