refactor(allocator): re-order code

[overlookmotel]

Follow-on after #11736. Pure refactor. Move all code related to AllocatorGuard to be together, so it's easier to read. Also rearrange std imports.

[overlookmotel]

• perf(allocator): reduce operations while Mutex lock is held in AllocatorPool #11761
• perf(allocator): remove Arc from AllocatorPool #11760
• refactor(allocator): re-order code #11759 👈 (View in Graphite)
• main

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[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #11759 will not alter performance

_{Comparing 06-16-refactor_allocator_re-order_code (01e52bc) with main (38dc614)}

Summary

✅ 38 untouched benchmarks

[camc314]

Merge activity

• Jun 16, 9:23 PM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Jun 16, 9:23 PM UTC: camc314 added this pull request to the Graphite merge queue.
• Jun 16, 9:30 PM UTC: Merged by the Graphite merge queue.