perf: Optimize string_to_array for scalar args

[neilconway]

Which issue does this PR close?

• Closes Optimize string_to_array for scalar args #21129.

Rationale for this change

When the delimiter (and null string, if supplied) are scalars, we can implement string_to_array more efficiently. In particular, we can construct a memmem::Finder and use it to search for delimiters more efficiently.

This PR implements this optimization; it also fixes a place where we were allocating an intermediate String for every character when the delimiter is NULL. (This isn't a common case but worth fixing.)

Benchmarks (M4 Max):

  single_char_delim/5:    34.8 µs  (was  61.1 µs)  -43%
  single_char_delim/20:  145.1 µs  (was 220.7 µs)  -34%
  single_char_delim/100: 679.4 µs  (was   1.04 ms) -35%

  multi_char_delim/5:    41.7 µs  (was  56.7 µs)  -27%
  multi_char_delim/20:  158.9 µs  (was 185.1 µs)  -14%
  multi_char_delim/100: 731.4 µs  (was 858.3 µs)  -15%

  with_null_str/5:    43.1 µs  (was  68.7 µs)  -37%
  with_null_str/20:  179.3 µs  (was 244.3 µs)  -27%
  with_null_str/100: 895.8 µs  (was   1.16 ms) -23%

  null_delim/5:    17.4 µs  (was  64.1 µs)  -73%
  null_delim/20:   63.0 µs  (was 233.4 µs)  -73%
  null_delim/100: 280.2 µs  (was   1.12 ms) -75%

  columnar_delim/5:    65.2 µs  (was  60.2 µs)  +8%
  columnar_delim/20:  217.2 µs  (was 224.1 µs)  -3%
  columnar_delim/100:   1.02 ms  (was   1.05 ms) -3%

What changes are included in this PR?

• Add benchmark for string_to_array
• Implement optimizations described above
• Refactor columnar (fallback) path to get rid of a lot of type dispatch boilerplate
• Improve SLT test coverage for the "columnar string, scalar other-args" case

Are these changes tested?

Yes.

Are there any user-facing changes?

No.

[neilconway]

I had to move some code around to group the string_to_array functions together in the file, sorry for the noisy diff.

[neilconway]

This was the inefficient NULL delimiter handling mentioned in the PR summary.