[DEAD] Improve SkipLast

[Orace]

Branch to fix morelinq#626.

We can avoid to consume the full sequence.
We only need O(skipCount) in memory.
The proposed implementation will not fail on an infinite sequence.

[Orace]

Closed because CountDown does not store all the element, so it's optimal in memory.
Anyway the current implementation is not really optimal (intermediate object creation, test for null, etc...)
I think that the MoreLinq methods should not call other MoreLinq methods but go straightforward to the most efficient implementation.

[Orace]

Reopened because we want efficient stuffs for MoreLinq

[atifaziz]

Could you please submit this PR on the main project/repo for reasons laid out in my comment on PR #8? Closing it here meanwhile.

Thanks!

[Orace]

Sorry about that, I forgot I was one level deeper ;)