refactor: Keep edge condvar on the stack

[Veykril]

Removes an unnecessary heap allocation when recording a dependency edge

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[codspeed-hq]

CodSpeed Performance Report

Merging #773 will not alter performance

_{Comparing Veykril:veykril/push-nmzmtvrqwwuv (b12d5f5) with master (296a8c7)}

Summary

✅ 12 untouched benchmarks

[Veykril]

Okay I think this does confirm that the benches are really sensitive to allocation changes. This should not regress performance whatsoever.

[MichaReiser]

I'm a bit concerned about the amount of unsafe code in salsa. But maybe that's just me because I'm not used to writing unsafe code. Avoiding an allocation in add_edge certainly is nice. But it's not entirely clear when we think that using unsafe is justified