C++ refactoring: ak.unflatten

[ioanaif]

No description provided.

[codecov]

Codecov Report

Merging #1360 (f8dac2f) into main (b2fd2be) will decrease coverage by 0.66%.
The diff coverage is 59.54%.

❗ Current head f8dac2f differs from pull request most recent head 1174b6a. Consider uploading reports for the commit 1174b6a to get more accurate results

Impacted Files	Coverage Δ
src/awkward/_v2/_connect/cling.py	0.00% <ø> (ø)
src/awkward/_v2/_lookup.py	97.50% <0.00%> (ø)
src/awkward/_v2/_prettyprint.py	66.09% <0.00%> (+2.29%)	⬆️
src/awkward/_v2/_typetracer.py	69.14% <0.00%> (ø)
src/awkward/_v2/forms/bitmaskedform.py	77.52% <0.00%> (-0.53%)	⬇️
src/awkward/_v2/forms/bytemaskedform.py	76.82% <0.00%> (-0.51%)	⬇️
src/awkward/_v2/forms/emptyform.py	74.60% <0.00%> (-5.40%)	⬇️
src/awkward/_v2/forms/form.py	81.87% <0.00%> (-8.20%)	⬇️
src/awkward/_v2/forms/indexedform.py	79.26% <0.00%> (-0.74%)	⬇️
src/awkward/_v2/forms/indexedoptionform.py	79.01% <0.00%> (-0.72%)	⬇️
... and 171 more

[jpivarski]

Great! I'll click the box that fixes the typo in the spelling of is_UnknownType.

[jpivarski]

Wrapping this up as a transform function that is only ever used once is now superfluous, but not harmful. In v1, it was needed so that the same procedure could be applied to each partition, but now we don't do PartitionedArrays anymore.

I also don't see a strong motivation to refactor it.