[mypyc] Faster dict iteration

[ilevkivskyi]

Fixes mypyc/mypyc#167

The implementation is pretty straightforward and follows the idea proposed in the issue. The perf impact is actually pretty small, around 1%

[ilevkivskyi]

Oh, tons of tests fail on Python 3.5 because this apparently changes dict iteration order there. It is probably a good opportunity to fix them (i.e. make them stable).

[ilevkivskyi]

@msullivan I think this should be now ready for review. Some notes:

• Python 3.5 turned out to be a pain (because of dict ordering), so I just skip this on Python 3.5.
• I squeeze some ms by having dedicated ops for keys/values/items, if you think it is not worth it, I can return to just one.
• Would it make sense to also add ops for keys/values/items when the appear in a non-loop context (there are pretty many of these in mypy)? If yes I can either cp them here or make a separate PR.

[JukkaL]

FWIW, I believe that the most common uses of keys/values/items outside of a loop context in general Python code is as arguments to list (and maybe set and/or dict), such as list(foo.values()). If special casing helps these use cases (at least for small dictionaries), I think that it's worth supporting non-loop contexts. Separate PR would be fine.

[JukkaL]

Nice, this will speed up some very common operations. In a simple microbenchmark this made iteration about 30% faster. Left a few minor comments.

[ilevkivskyi]

@msullivan So I disable the fast path for subclasses now (and enable on Python 3.5). It looks pretty complicated, but not too bad probably. Please let me know if it looks OK.

[JukkaL]

This looks good to me. Let's merge this tomorrow unless @msullivan has some additional feedback.

[ilevkivskyi]

This looks good to me. Let's merge this tomorrow unless @msullivan has some additional feedback.

I am assuming Sully doesn't have any comments here, so will merge now.