Implement as_chunks with split_at_unchecked

[scottmcm]

We were discussing various ways to do this on Discord, and in the process I noticed that https://rust.godbolt.org/z/1P16P37Go is emitting a panic path inside as_chunks. It optimizes out in release, but we could just not do that in the first place.

We're already doing unsafe code that depends on this value being calculated correctly, so might as well call split_at_unchecked instead of split_at.

[rustbot]

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[Noratrieb]

this is a good PR

[bors]

📌 Commit 49af347 has been approved by Nilstrieb

It is now in the queue for this repository.

[bors]

⌛ Testing commit 49af347 with merge ee9a9f8...

[bors]

☀️ Test successful - checks-actions
Approved by: Nilstrieb
Pushing ee9a9f8 to master...

[rust-timer]

Finished benchmarking commit (ee9a9f8): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.3%, 2.3%]	1
Regressions ❌ (secondary)	2.4%	[2.1%, 2.6%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.3%	[2.3%, 2.3%]	1

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 675.036s -> 675.709s (0.10%)
Artifact size: 315.79 MiB -> 315.74 MiB (-0.02%)