Use C string literals & deprecate `c_str!`

Redo of !624 (closed) based on comments.

Besides C string literals, I fixed a few trivial Clippy lints that don't affect anything.

I also found a test that doesn't compile (unrelated to my changes), but I just marked it as a fix-me for later.

Draft while I recompile everything overnight and test tomorrow to ensure nothing broke.

mentioned in merge request !624 (closed)

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marked this merge request as ready

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mentioned in commit de56c143

merged