String module spannification

[jwosty]

Description

Some String module perf improvements (heap allocation reduction) on .NET Standard 2.1+

Part of #20283

Checklist

• Test cases added
• Performance benchmarks added in case of performance changes
• Release notes entry updated: pending

[github-actions]

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[jwosty]

String.filter seems to be tricky since the output length is unknown and we can't invoke the predicate multiple times.

The best I could come up with after a few attempts is swapping out the intermediate array for a stackalloc, and this indeed tamps down allocations quite a bit, we just need to figure out what a good upper limit (on stackalloc size) would be.

Tried some "clever" approaches with BitArray as a two-pass intermediate but couldn't find any wins there

Honestly not sure how much more clever it can get outside a careful stackalloc -- would love to hear ideas