arrow

ARROW-10557: [C++] Add scalar string slicing/substring extract kernel

Needs a rebase after https://github.com/apache/arrow/pull/8621 is merged

I totally agree with https://github.com/python/cpython/blob/c9bc290dd6e3994a4ead2a224178bcba86f0c0e4/Objects/sliceobject.c#L252

This was tricky to get right, the main difficulty is in manually dealing with reverse iterators. Therefore I put on extra guardrails by having the Python unittests cover a lot of cases. All edge cases detected by this are translated to the C++ unittest suite, so we could reduce them to reduce pytest execution cost (I added 1 second).

Slicing is based on Python, `[start, stop)` inclusive/exclusive semantics, where an index refers to a codeunit (like Python apparently, badly documented), and negative indices start counting from the right. `step != 0` is supported, like Python.

The only thing we cannot support easily, are things like reversing a string, since in Python one can do `s[::-1]` or `s[-1::-1]`, but we don't support empty values with the Option machinery (we model this as an c-`int64`). To mimic this, we can do `pc.utf8_slice_codeunits(ar, start=-1, end=-sys.maxsize, step=-1)` (i.e. a very large negative value).

For instance, libraries such as Pandas and Vaex can do sth like that, confirmed to be working by modifying the unittest like this:
```python
import sys
@pytest.mark.parametrize('start', list(range(-6, 6)) + [None])
@pytest.mark.parametrize('stop', list(range(-6, 6)) + [None])
@pytest.mark.parametrize('step', [-3, -2, -1, 1, 2, 3])
def test_slice_compatibility(start,stop, step):
    input = pa.array(["", "𝑓", "𝑓ö", "𝑓öõ", "𝑓öõḍ", "𝑓öõḍš"])
    expected = pa.array([k.as_py()[start:stop:step] for k in input])
    if start is None:
        start = -sys.maxsize if step > 0 else sys.maxsize
    if stop is None:
        stop = sys.maxsize if step > 0 else -sys.maxsize
    result = pc.utf8_slice_codeunits(input, start=start, stop=stop, step=step)
    assert expected.equals(result)
```

So libraries using this can implement the full Python behavior with this workaround.

Closes #9000 from maartenbreddels/ARROW-10557

Lead-authored-by: Maarten A. Breddels <maartenbreddels@gmail.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>