C++ refactoring: Implementing combinations

[findingmochi]

This PR implements the _combinations function for all the Awkward Array types except VirtualArray.

• _combinations implementation

• BitMaskedArray
• ByteMaskedArray
• EmptyArray
• IndexedArray
• IndexedOptionArray
• ListArray
• ListOffsetArray
• NumpyArray
• RecordArray
• RegularArray
• UnionArray
• UnmaskedArray

• _combinations tests

• BitMaskedArray
• ByteMaskedArray
• EmptyArray
• IndexedArray
• IndexedOptionArray
• ListArray
• ListOffsetArray
• NumpyArray
• RecordArray
• RegularArray
• UnionArray
• UnmaskedArray

[jpivarski]

We talked on Slack (code reviews are much better when interactive, but unfortunately, we don't get a public log). Some of the things I was worried about were unfounded (declaring the Index as np.intp is exactly the right thing to do, and we do in fact need that ptr property, at least for this exceptional double-pointer case). There were only a few remaining items in an overall excellent PR.

• centralize the n < 1 check in Content.combinations, so that it doesn't have to be in every _combinations method
• see if ListArray._combinations can reuse ListOffsetArray._combinations by calling it with itself (its self) as the self argument, and if so, do so
• add a pytestmark to skip the tests if Python 2

That's it! When you're done, let me know that you're not planning on committing any more changes and I'll merge it when the tests pass.

Thanks again; this is great!

[findingmochi]

I am done with all the changes, so you can merge this, after the tests pass. Thank You!