C++ refactoring: flatten()

[ioanaif]

No description provided.

[codecov]

Codecov Report

Merging #1148 (235476f) into main (f795658) will increase coverage by 1.07%.
The diff coverage is 76.80%.

Impacted Files	Coverage Δ
src/awkward/_v2/forms/bitmaskedform.py	75.64% <ø> (ø)
src/awkward/_v2/forms/bytemaskedform.py	76.05% <ø> (ø)
src/awkward/_v2/forms/emptyform.py	78.84% <ø> (+1.92%)	⬆️
src/awkward/_v2/forms/indexedoptionform.py	76.81% <ø> (ø)
src/awkward/_v2/forms/listform.py	75.94% <ø> (ø)
src/awkward/_v2/forms/listoffsetform.py	80.55% <ø> (ø)
src/awkward/_v2/forms/recordform.py	66.46% <ø> (ø)
src/awkward/_v2/forms/regularform.py	75.34% <ø> (ø)
src/awkward/_v2/forms/unionform.py	76.19% <ø> (ø)
src/awkward/_v2/forms/unmaskedform.py	74.57% <ø> (ø)
... and 67 more

[jpivarski]

I reviewed it all carefully, and it looks great. Thanks!