[Issue 17360] std.range.only doesn't allow ref access

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=17360

--- Comment #1 from John Colvin <john.loughran.colvin at gmail.com> ---
Adding `auto ref` to `front`, `back` and `opIndex` is an obvious fix but falls
foul of https://issues.dlang.org/show_bug.cgi?id=17512

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