[red-knot] Use BitSet::union for merging of declarations

[sharkdp]

Summary

In SymbolState merging, use BitSet::union instead of inserting declarations one by one. This used to be the case but was changed in #15019 because we had to iterate over declarations anyway.

This is an alternative to #15419 by @MichaReiser, just to see which is better in terms of performance.

Codspeed results

There's almost no difference in performance (note that the baseline also changed):

Test Plan

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[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[MichaReiser]

Thanks. This is the proper solution which I think is also easier to understand (it's clear that the order isn't relevant).

[AlexWaygood]

nit that is probably not even worth a followup PR: I'd have probably called this something like union_with (similar to https://docs.rs/bit-set/latest/bit_set/struct.BitSet.html#method.union_with) since union methods on other structs seem to generally return a new value rather than working in-place (https://doc.rust-lang.org/std/collections/struct.HashSet.html#method.union, https://docs.rs/bitflags/latest/bitflags/trait.Flags.html#method.union, https://docs.rs/bit-set/latest/bit_set/struct.BitSet.html#method.union)

[sharkdp]

Thanks. I just put some previously-approved code back in place that was there before, which is why I didn't wait for any other reviews. But that seems like a reasonable improvement.