[Suggestion] Make another Combination iterator

[fyrchik]

Current implementation makes it hard to iterate over all combinations of infinite iterator.
E.g. every element from (1..).combinations(3) will always start with [1,2,...] so there is no way to ever get [2,3,4].

But there is another way to implement this:

1. Iterate over all combinations which uses only indices < n (in lexicographic order).
2. Iterate over the rest.

This way our example will be [1,2,3], [1,2,4], [1,3,4], [2,3,4], [1,2,5], ...

It is practical to also have this implementation alongside with current.
I can make a PR.

EDIT: This is similar to #293 (but not the same).