chore(rust): Factor out ensure_is_constant_duration

[MarcoGorelli]

This would enable deduping a few things in https://github.com/pola-rs/polars/pull/12597/files

[codspeed-hq]

CodSpeed Performance Report

Merging #15733 will improve performances by 14.83%

_{Comparing MarcoGorelli:factor-ensure-is-constant-duration-out (55aab11) with main (252e4cf)}

Summary

⚡ 1 improvements
✅ 21 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	MarcoGorelli:factor-ensure-is-constant-duration-out	Change
⚡	test_sort_nan_1942	1,139.6 µs	992.4 µs	+14.83%

[stinodego]

Extracting this logic is a good move if it will be reused 👍 Though I would generally expect that as part of the PR that actually introduces the duplication. After this PR I could look at the code base and think: let's remove this method as it's only used in 1 place. So in this case I would recommend to just include this in the PR you linked.

[MarcoGorelli]

thanks for your review!

Though I would generally expect that as part of the PR that actually introduces the duplication

That's a fair point - in this case, the discussion in 12597/files has been going on for a while, so I didn't really want to clutter it too much

[stinodego]

Good to go if CI is green!