feat: into_struct_fields_*

[danking]

I find myself wanting this in Spiral. Is it reasonable?

[a10y]

why is the as_struct_fields version not good enough here?

[danking]

We maybe shouldn't do this but there's cases where I get an owned DType and I want to convert to a Spiral Schema which is a StructFields without nullability (b/c its a SQL-style table: no top-level nulls).

[danking]

I currently do as_struct_fields(...).cloned()

[codspeed-hq]

CodSpeed Performance Report

Merging #5128 will not alter performance

_{Comparing dk/into-struct-fields (3ae901a) with develop (e1ca80a)}

Summary

✅ 1325 untouched
⏩ 99 skipped¹

Footnotes

1. 99 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[codecov]

Codecov Report

❌ Patch coverage is 0% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.76%. Comparing base (8945cd8) to head (3ae901a).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
vortex-dtype/src/dtype.rs	0.00%	33 Missing ⚠️

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[a10y]

I think this is fine, maybe we should add owned into_ variants for the other conversion methods too

[danking]

I added an into_ method for each of the per-variant as_ methods.

[danking]

all checks passing now.