rustdoc: reduce DocFragment::indent to u8

[lolbinarycat]

Estimating this should save 52kB when documenting libcore, and more for larger libraries. This saves a whole 2 pointers worth per individual doc comment, and only costs not supporting completely absurd amounts of indentation.

[lolbinarycat]

@bors try @rust-timer queue profiles=doc

[rust-bors]

☀️ Try build successful (CI)
Build commit: a065d2c (a065d2c5f9140ca4becde2e7957b95c3fe5d4866)
Base parent: 9a9d194 (9a9d1946d1e119676b7a6a8a26a264061e61027d)

[rust-timer]

Finished benchmarking commit (a065d2c): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This perf run didn't have relevant results for this metric.

Max RSS (memory usage)

Results (primary 2.5%, secondary 2.3%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.5%	[2.2%, 2.7%]	2
Regressions ❌ (secondary)	2.3%	[2.0%, 2.6%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.5%	[2.2%, 2.7%]	2

Cycles

This perf run didn't have relevant results for this metric.

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 489.957s -> 489.243s (-0.15%)
Artifact size: 407.04 MiB -> 406.28 MiB (-0.19%)

[lolbinarycat]

no idea why making the struct smaller makes memory usage go up. what.