Even and Odd Integers

[Letan (Letanyan Arumugam)]

This isn't in any guidelines for including something in the Standard Library, but I think it would be helpful.

@xwu

1. Does the proposed addition solve a need that is common across multiple different problem domains?
2. Does it significantly improve readability?
3. Is it important to have a consistent way of doing this across all Swift codebases?
4. Is there a correctness risk to existing alternatives that the new addition would avoid?
5. Can the new addition be made more efficient than existing alternatives?

I think you got 1 and 2. Maybe 5, but a compiler should probably handle that case.