icu_provider_blob contains code that will be rejected by a future version of Rust

[sffc]

I landed this code in #4887; I had intended to discuss it during the review, but better late than never. I would appreciate if someone else with a high degree of Rust fluency (likely @Manishearth) could see if there's a way to make this work if the #![allow(semicolon_in_expressions_from_macros)] in auxkey_bench.rs is removed.

Reduced example:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=75617c4dc910f1e7a1f044dbee8e6c9c

[sffc]

Fix in #4911