C++ refactoring: utility methods for high-level functions in v2.

[jpivarski]

No description provided.

[codecov]

Codecov Report

Merging #1132 (c2e03bb) into main (0b0fb3f) will increase coverage by 0.56%.
The diff coverage is 63.49%.

Impacted Files	Coverage Δ
src/awkward/_v2/behaviors/string.py	100.00% <ø> (ø)
src/awkward/_v2/forms/bitmaskedform.py	67.94% <ø> (ø)
src/awkward/_v2/forms/bytemaskedform.py	69.01% <ø> (ø)
src/awkward/_v2/forms/indexedform.py	72.00% <ø> (+8.84%)	⬆️
src/awkward/_v2/forms/unmaskedform.py	69.49% <ø> (ø)
src/awkward/_v2/highlevel.py	41.37% <ø> (+6.26%)	⬆️
.../awkward/_v2/operations/convert/ak_from_buffers.py	80.00% <0.00%> (ø)
src/awkward/_v2/operations/convert/ak_from_cupy.py	80.00% <0.00%> (ø)
src/awkward/_v2/operations/convert/ak_from_iter.py	80.00% <0.00%> (ø)
src/awkward/_v2/operations/convert/ak_from_jax.py	80.00% <0.00%> (ø)
... and 121 more

[jpivarski]

recursively_apply and broadcast_and_apply haven't been fully tested, but that will come when we reintroduce all the high-level functions. Whatever bugs are left are corner-cases. (It's translated from old code that worked and the basic cases work in the new code.)

So I'll merge this as soon as the tests pass, and then move on to to/from_buffers.