Allow creation of tracked associated functions (without self)

[regexident]

Fixes #858 & #585.

This allows users to annotate impl blocks with #[salsa::tracked] and then create tracked associated type-level functions by marking individual functions with #[salsa::tracked].

Note this requires your code that looks like:

#[salsa::tracked]
impl MyOutput {
    #[salsa::tracked]
    fn tracked_fn(db: &dyn Db, input: MyInput) -> MyOutput {
        MyOutput::new(db, input.field(db) * 2)
    }
}

You get an error if you annotate an associated function with #[salsa::tracked] but forget to mark the impl block.

[netlify]

✅ Deploy Preview for salsa-rs canceled.

Name	Link
🔨 Latest commit	51bdd88
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[codspeed-hq]

CodSpeed Performance Report

Merging #859 will improve performances by 4.03%

_{Comparing regexident:tracked-assoc-fns (51bdd88) with master (678f51a)}

Summary

⚡ 1 improvements
✅ 11 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	amortized[SupertypeInput]	3.8 µs	3.6 µs	+4.03%

[Veykril]

Needs a rebase to fix CI I believe