Adapt code to support -preview=in

[Geod24]

When the switch is provided, '[auto] ref in' becomes illegal in the parser. This little trick, using static if + mixin, allow to have a different function based on whether the switch is used or not. 

If I could get a point release, that would allow me to make progress with dlang/dmd#11632

[codecov-commenter]

Codecov Report

Merging #318 into master will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@ ## master #318 +/- ## ======================================= Coverage 92.39% 92.39% ======================================= Files 59 59 Lines 13064 13064 ======================================= Hits 12071 12071 Misses 993 993 

Impacted Files	Coverage Δ
source/mir/ndslice/internal.d	72.00% <ø> (ø)

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[9il]

Thank you for the PR. Could you please describe why auto ref in can't be replaced with auto ref scope const?

[Geod24]

It can. I just figured you actually wanted to use in as it was intended here, so I went the extra way to make it work.
But I can just revert it and simply remove in in favor of scope const.

[9il]

But I can just revert it and simply remove in in favor of scope const.

Yes, please do so. It is preferable

[Geod24]

Updated