feature: access underlying IngredientImpl in interned structs

[davidbarsky]

(Builds atop of #620 and #618).

This PR adds the ability to fetch interned data from the Salsa database by simply knowing the name of the interned struct and the corresponding ID. For example:

#[salsa::interned_sans_lifetime(id = salsa::Id)]
struct InternedString {
    data: String
}

let s1 = InternedString::new(db, "Hello, world!");

// later...

let interned_s1 = InternedString::ingredient(&db).fields(&db, s1).0;

[netlify]

✅ Deploy Preview for salsa-rs canceled.

Name	Link
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[codspeed-hq]

CodSpeed Performance Report

Merging #623 will not alter performance

_{Comparing davidbarsky:davidbarsky/access-underlying-ingredientimpl-in-interned-structs (1612f16) with master (3c7f169)}

Summary

✅ 9 untouched benchmarks

[davidbarsky]

Closed in favor of #632.