refactor(allocator/vec): access len and cap fields via getters/setters

[overlookmotel]

Follow-on after #10884.

Pure refactor.

To make it explicit when we're converting u32 to/from usize:

• Make RawVec's len field private.
• Make all access to len and cap fields go via getter/setter methods.
• Name those methods *_u32 and *_usize so it's clear when a conversion is happening.

I also renamed cap_* methods to capacity_* for consistency with Vec::capacity.

I haven't used these methods in RawVec because there's some subtlety around when you want the "raw" cap value, and when you want the value capacity method returns, which is different when T is a ZST. Hopefully we can remove support for ZSTs, and remove this ambiguity.

[overlookmotel]

• perf(allocator/vec): reduce pointer maths in Vec::extend_desugared #11087
• perf(allocator/vec): Vec::extend_desugared set len only at end #11082
• refactor(allocator/vec): access len and cap fields via getters/setters #11081 👈 (View in Graphite)
• refactor(allocator/vec): re-order methods #11080
• perf(allocator/vec): remove SetLenOnDrop #11079
• refactor(allocator/vec): limit scope of #[expect(clippy::cast_possible_truncation)] #11074
• perf(allocator/vec): remove alloc_guard from RawVec::from_raw_parts_in + clarify safety docs #11073
• docs(allocator/vec): correct safety comments for RawVec::append_elements #11072
• refactor(allocator/vec): clarify comment #11071
• main

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[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #11081 will improve performances by 14.32%

_{Comparing 05-16-refactor_allocator_vec_access_len_and_cap_fields_via_getters_setters (8ec0c74) with main (df4cc8d)}

Summary

⚡ 2 improvements
✅ 36 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	mangler[RadixUIAdoptionSection.jsx]	14.4 µs	13.7 µs	+4.74%
⚡	mangler[cal.com.tsx]	3.5 ms	3 ms	+14.32%

[Boshen]

Merge activity

• May 19, 9:42 PM EDT: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• May 19, 9:42 PM EDT: Graphite disabled "merge when ready" on this PR due to: a merge conflict with the target branch; resolve the conflict and try again..
• May 19, 9:42 PM EDT: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• May 19, 10:03 PM EDT: Boshen added this pull request to the Graphite merge queue.
• May 20, 2:04 AM UTC: The Graphite merge queue couldn't merge this PR because it had conflicts with the trunk branch.
• May 19, 10:06 PM EDT: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• May 20, 2:06 AM UTC: The merge label '0-merge' was removed. This PR will no longer be merged by the Graphite merge queue
• May 20, 8:30 PM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• May 20, 8:30 PM UTC: overlookmotel added this pull request to the Graphite merge queue.
• May 20, 8:39 PM UTC: Merged by the Graphite merge queue.