perf: Optimize substr for Utf8, LargeUtf8

[neilconway]

Which issue does this PR close?

• Closes Optimize substr() to avoid copying for Utf8, LargeUtf8 #21364.

Rationale for this change

For Utf8 and LargeUtf8 inputs, we can optimize substr to avoid copying the output strings; instead, we can return a StringViewArray that points into the input value buffer.

Benchmarks (M4 Max):

Array args, no count, short strings (strlen=12):

• string_view: 25.8 → 25.2 (-2.5%)
• string: 32.9 → 23.8 (-27.9%)
• large_string: 35.0 → 24.0 (-31.4%)

Array args, with count, long strings (count=64, strlen=128):

• string_view: 41.5 → 41.9 (+1.0%)
• string: 47.4 → 37.0 (-22.0%)
• large_string: 47.6 → 37.5 (-21.2%)

Array args, short count, long strings (count=6, strlen=128):

• string_view: 48.8 → 48.7 (-0.2%)
• string: 59.5 → 49.1 (-17.6%)
• large_string: 65.1 → 49.2 (-24.4%)

Scalar start, no count, short strings (strlen=12):

• string_view: 26.3 → 25.9 (-1.3%)
• string: 32.8 → 24.3 (-25.9%)

Scalar start, no count, long strings (strlen=128):

• string_view: 41.4 → 41.7 (+0.7%)
• string: 47.2 → 37.2 (-21.2%)

Scalar start=1, no count, long strings (strlen=128):

• string_view: 42.0 → 41.9 (-0.2%)
• string: 47.7 → 37.3 (-21.8%)

Scalar args, short strings (count=6, strlen=12):

• string_view: 47.3 → 47.8 (+1.1%)
• string: 61.1 → 50.4 (-17.5%)

Scalar args, long strings (count=64, strlen=128):

• string_view: 42.4 → 42.6 (+0.6%)
• string: 47.7 → 37.8 (-20.8%)
• large_string: 51.5 → 38.4 (-25.5%)

What changes are included in this PR?

• Implement optimization
• Other minor code cleanup
• Add a benchmark (only somewhat related to this optimization but related to future optimization work)

Are these changes tested?

Yes.

Are there any user-facing changes?

No.