feature: support interned structs without lifetimes

[davidbarsky]

For background context, see Zulip, but the long-and-short of it is that the 'db lifetime on interned values made porting rust-analyzer to the new Salsa nearly impossible. However, I realized that I could simply get rid of the 'db lifetime on interned structs, hence this PR.

I also ported over the interned-struct tests.

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[codspeed-hq]

CodSpeed Performance Report

Merging #618 will not alter performance

_{Comparing davidbarsky:davidbarsky/interning-without-lifetimes (60ca556) with master (3c7f169)}

Summary

✅ 9 untouched benchmarks

[MichaReiser]

I don't mind this, but I would prefer to have all the backwards compat macros behind an off-by-default feature flag to prevent new users from starting to use them.

[davidbarsky]

Closed in favor of #632.