Bug 17360 – std.range.only doesn't allow ref access

This a pain because it means things like this don't work: int[] a = [1,2,3]; only(a).front.popFront(); Error: template std.range.primitives.popFront cannot deduce function from argument types !()(int[]), candidates are: std/range/primitives.d(2084): std.range.primitives.popFront(T)(ref T[] a) if (!isNarrowString!(T[]) && !is(T[] == void[])) std/range/primitives.d(2107): std.range.primitives.popFront(C)(ref C[] str) if (isNarrowString!(C[]))

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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