Use Peekable in lexer_unescape

[hkBst]

Use a Peekable<CharIndices<'_>> instead of going back and forth between string slice and chars iterator.

• this gets rid of most position computations
• allows removal of double traversal for correct backslash newline escapes in skip_ascii_whitespace

Improves documentation

r? @nnethercote

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[hkBst]

This is an off-shoot of #136538 separating just the introduction of Peekable. This is likely also a perf regression...

[nnethercote]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 43b0a00 with merge c873a97...

[bors]

☀️ Try build successful - checks-actions
Build commit: c873a97 (c873a97a97bc25fdce9305797177509a3bea45b0)

[rust-timer]

Finished benchmarking commit (c873a97): comparison URL.

Overall result: ❌ regressions - please read the text below

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.2%, 0.6%]	12
Regressions ❌ (secondary)	0.8%	[0.5%, 1.4%]	16
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[0.2%, 0.6%]	12

Max RSS (memory usage)

Results (primary 2.7%, secondary 2.9%)

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)	2.7%	[2.1%, 4.7%]	13
Regressions ❌ (secondary)	3.2%	[2.3%, 5.4%]	19
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.1%	[-3.1%, -3.1%]	1
All ❌✅ (primary)	2.7%	[2.1%, 4.7%]	13

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: 790.14s -> 789.165s (-0.12%)
Artifact size: 347.71 MiB -> 347.71 MiB (0.00%)