[Issue 21216] SortedRange.empty should be const, .front should be inout

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21216

--- Comment #1 from Simon Naarmann <eiderdaus at gmail.com> ---
Discussion on the D Forums:
https://forum.dlang.org/thread/hiuncwrfbpouoluidsyb@forum.dlang.org

Gist: Instead of adding const/inout, which might not fit every wrapped range,
we should use `template this` to let the compiler figure out the appropriate
annotations for empty/front/... individually for each wrapped range.

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