combinations, partitions functions should return iterators

[stevengj]

See this mailing-list discussion.

As @StefanKarpinski notes, the combinations function should really return an Iterator type; the task-based interface was doubtless easier to implement but is harder to use.

Furthermore, it looks like we have a little ways to go in order to beat the CPython itertools.combinations function.

[StefanKarpinski]

The partitions function should likewise be improved. We probably want iterable permutations and subsets functions as well.

[JeffBezanson]

Now we are significantly faster.

[stevengj]

Nice! But a hornéd hat of shame for not updating the documentation...

[JeffBezanson]

drat. thanks for the reminder.

[stevengj]

Is this now done? Can we close the issue and add a NEWS item?

[JeffBezanson]

The integer and set partitions functions haven't been converted yet.

[kmsquire]

I should be submittiing these in a few minutes.

[JeffBezanson]

Kevin you're a beast!