Add support for multiple values to std.range.only

Basically the non-dynamic-allocation version of `[values...]`, which is probably used a lot in the wild despite its performance implication.

I find myself resorting to a utility module including this functionality in more and more of my projects, which is why I've come to think it should be in Phobos (e.g. https://github.com/JakobOvrum/Dirk/blob/master/irc/util.d#L14).