C++ refactoring: Highlevel non-reducers and improved testing/fixes for reducers.

[jpivarski]

Tests that still need to be converted to v2:

• test_0889-ptp.py
• test_0982-missing-case-in-nonlocal-reducers.py
• test_0992-correct-ptp-unmasking.py
• test_1000-fixes-argmax-for-ListOffsetArray-with-nonzero-start.py
• test_1071-mask-identity-false-should-not-return-option-type.py
• test_1106-argminmax-axis-None-missing-values.py

[codecov]

Codecov Report

Merging #1206 (b3f563c) into main (1751ffc) will increase coverage by 0.35%.
The diff coverage is 77.00%.

Impacted Files	Coverage Δ
src/awkward/_v2/_broadcasting.py	90.59% <ø> (ø)
.../awkward/_v2/operations/structure/ak_singletons.py	80.00% <ø> (ø)
src/awkward/_v2/operations/convert/ak_to_list.py	50.00% <42.30%> (-30.00%)	⬇️
src/awkward/_v2/highlevel.py	64.99% <66.66%> (+0.21%)	⬆️
src/awkward/_v2/operations/reducers/ak_mean.py	78.57% <70.00%> (-1.43%)	⬇️
src/awkward/_v2/operations/reducers/ak_moment.py	78.57% <70.00%> (-1.43%)	⬇️
src/awkward/_v2/operations/convert/ak_to_numpy.py	88.88% <75.00%> (-11.12%)	⬇️
src/awkward/_v2/operations/reducers/ak_covar.py	82.35% <76.92%> (+2.35%)	⬆️
src/awkward/_v2/operations/reducers/ak_var.py	82.35% <76.92%> (+2.35%)	⬆️
src/awkward/_v2/operations/reducers/ak_corr.py	81.81% <77.77%> (+1.81%)	⬆️
... and 22 more