Suggestion: Operator `in` for slices

[Ola Fosheim Grøstad]

On Monday, 20 December 2021 at 14:21:08 UTC, Paul Backus wrote:
> ```d
> import std.algorithm.searching;
>
> if (["if", "while", "for"].canFind(keyword)) {
>     // ...
> }
> ```

You can always create a library construct, but then you should 
also argue in favour of removing AAs and "in". You either do it, 
or you don't. Why the half-measures?