feature: allow overriding interned_sans_lifetime-annotated structs with custom IDs

[davidbarsky]

(This feature builds atop of #618, but GitHub doesn't do git range-diff between PRs, so...)

This PR adds support for replacing the salsa::Id in #[salsa::interned_sans_lifetime]-annotated structs with a newtype that implements salsa::plumbing::FromId and salsa::plumbing::AsId. I expect that this would simplify migrating rust-analyzer, especially inside of the span crate, as we use bit-packed u32s.

[netlify]

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[codspeed-hq]

CodSpeed Performance Report

Merging #620 will not alter performance

_{Comparing davidbarsky:davidbarsky/allow-overrriding-ids-in-lifetimeless-interned-structs (35fd2af) with master (3c7f169)}

Summary

✅ 9 untouched benchmarks

[davidbarsky]

Closed in favor of #632.