implement Iterator::nth for Combinations

[fyrchik]

Hello!
I have implemented nth method for Combinations iterator.

Closes #301 .
Let me know, what do you think. Other changes in tests are due to rustfmt.

[fyrchik]

I have rebased this on master.

[jswrenn]

Thanks! I'll take a look at this soon.

[jswrenn]

@fyrchik I've filed a PR adding a quickcheck test on your feature branch: fyrchik#1

That test case is finding differences between the output of your specialized Combinations::nth and the current unspecialized method.

The specialized version of Combinations::nth needs to behave identically to the current unspecialized method before I can merge this PR.

[fyrchik]

I have reimplemented this and got rid of binomial coefficients.
We are still advancing iterator one-by-one, however there are no intermediate allocations.
I have also added quickcheck tests from @jswrenn to be sure that new implementations behaves similar to the old.

[jswrenn]

Thanks! Could you resolve the merge conflict? Also, note that the behavior of combinations(0) changed in #383.

[fyrchik]

I have rebased on master. cargo +nightly test finished successfully.

[Philippe-Cholet]

Hi @fyrchik, Combinations has evolved a bit since you worked on this.
Do you want to update your work here? I would promptly respond/review.
We now have tests about specializations so I don't think we need your tests.

EDIT: Well, I see all your repositories are archived, I'm closing this.
I'll probably make this myself when I find more time unless someone does this before me.