Fix flatten array with sliced input

[kylebarron]

We should add a test for this before merging

In [1]: import pyarrow as pa

In [2]: from arro3.compute import list_flatten

In [3]: list_arr = pa.array([[1, 2], [3, 4]])

In [4]: pa.array(list_flatten(list_arr))
Out[4]:
<pyarrow.lib.Int64Array object at 0x107390160>
[
  1,
  2,
  3,
  4
]

In [5]: sliced = list_arr.slice(1, 2)

In [6]: pa.array(list_flatten(sliced))
Out[6]:
<pyarrow.lib.Int64Array object at 0x106f927a0>
[
  3,
  4
]

In [7]: sliced = list_arr.slice(0, 1)

In [8]: pa.array(list_flatten(sliced))
Out[8]:
<pyarrow.lib.Int64Array object at 0x10701c400>
[
  1,
  2
]

[kylebarron]

Oh struct_field would have the same problem with sliced input...

[kylebarron]

Ref: apache/arrow-rs#6151