Prepare for a breaking change in the Rust compiler.

[oli-obk]

The soundness fix in rust-lang/rust#115008 will cause rs-matter to break, even though it is not unsound. The missing bound is very hard to abuse, but still a soundness hole in our type system.

It will likely take 12 weeks before a stable compiler with the soundness fix is shipped.

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[oli-obk]

The change is trivial enough (and proposed by the Rust compiler, not actually written by me). It is not complex enough to need a license, so if you want you can just copy this code yourself without a license.

[ivmarkov]

@kedars We can close this one now that #99 is merged.