Add a skip_every iterator

[shssoichiro]

The name and semantics can be changed if necessary, but I'd like an iterator that skips every nth element, and I wasn't able to find an existing one in reading through the docs. For example:

let items = vec![1, 2, 3, 4, 5, 6, 7, 8];
assert_eq!(vec![1, 2, 4, 5, 7, 8], items.iter().skip_every(3).collect());

[fyrchik]

This is equivalent to .enumerate().filter(|(n,_)| n % 3 != 2).map(|(_,v)| v). (or just enumerate + filter_map. I am not sure if specific implementation will be faster.

[shssoichiro]

This ticket is from 3 years ago, so probably could be closed anyway. Thanks for alternative suggestion!