perf: Optimize split_part, support Utf8View (#21119)

## Which issue does this PR close?

- Closes #21117.
- Closes #21118 .

## Rationale for this change

`split_part` currently accepts `Utf8View` but always returns `Utf8`.
When given `Utf8View` input, it should instead return `Utf8View` output.

While we're at it, optimize `split_part` for single-character delimiters
(the common case): `str::split(&str)` is significantly slower than
`str::split(char)` for single-character ASCII delimiters, because the
former uses a general string matching algorithm but the latter uses
`memchr::memchr`.

Benchmark results (M4 Max):

  - `utf8_single_char/pos_first`: 142 µs → 104 µs (-26%)
  - `utf8_single_char/pos_middle`: 389 µs → 365 µs (-6%)
  - `utf8_single_char/pos_negative`: 154 µs → 109 µs (-29%)
  - `utf8_multi_char/pos_middle`: 356 µs → 361 µs (~0%, noise)
  - `utf8view_single_char/pos_first`: 143 µs → 111 µs (-22%)
  - `utf8_long_strings/pos_middle`: 2568 µs → 1984 µs (-23%)
  - `utf8view_long_parts/pos_middle`: 998 µs → 470 µs (-53%)

## What changes are included in this PR?

* Revise `split_part` benchmarks to reduce redundancy and improve
`Utf8View` coverage
* Support `Utf8View` -> `Utf8View` in `split_part`
* Refactor `split_part` to cleanup some redundant code
* Optimize `split_part` for single-character delimiters
* Add SLT test coverage for `split_part` with `Utf8View` input

## Are these changes tested?

Yes. New tests and benchmarks added.

## Are there any user-facing changes?

No.

Author: neilconway