[SR-9226] Breadcrumb benchmarks

[milseman]

Previous ID	SR-9226
Radar	None
Original Reporter	@milseman
Type	Sub-task
Status	Closed
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Sub-task
Assignee	@lorentey
Priority	Medium

md5: 093a1355c4a0693c829916647a30dc63

Parent-Task:

• SR-8905 Gaps in String benchmarking

Issue Description:

String's breacrumbing needs benchmarks, over both ASCII and mixed text, measuring:

• `getCharacters` via `_copyUTF16CodeUnits`

• `Range<String.Index>` to `Range<Int>` via `_toUTF16Offsets`

• `Range<Int>` to `Range<String.Index>` via `_toUTF16Indices`

• Int -> String.Index via `_toUTF16Index`

• String.Index -> Int via `_toUTF16Offset`

• String breadcrumb creation via a new entry point this benchmark needs to add.

[lorentey]

#20769