Rename low-level methods to match high-level function names.

[jpivarski]

There were fewer of these than I thought there was going to be.

• localindex → local_index
• validityerror → validity_error
• rpad → pad_none
• withparameter → with_parameter

I changed mid-level names (i.e. on the layouts) because that's a public interface for downstream developers. I didn't change any kernel function names because those are totally internal and can be changed at any time. Also, those wouldn't be across-the-board changes that affect mergeability. Also, also, the kernel names aren't very good as they are, but they're defined in terms of their function (the Python snippets in kernel-signatures.yml), rather than a meaningful name.

[codecov]

Codecov Report

Merging #1468 (c74ecda) into main (de7cae8) will decrease coverage by 0.47%.
The diff coverage is 70.22%.

Impacted Files	Coverage Δ
src/awkward/_v2/_connect/numexpr.py	88.40% <ø> (ø)
...c/awkward/_v2/_connect/rdataframe/to_rdataframe.py	0.00% <0.00%> (ø)
src/awkward/_v2/operations/ak_backend.py	10.00% <0.00%> (ø)
src/awkward/_v2/operations/ak_from_arrow.py	100.00% <ø> (ø)
src/awkward/_v2/operations/ak_from_arrow_schema.py	50.00% <ø> (ø)
src/awkward/_v2/operations/ak_from_buffers.py	86.13% <ø> (ø)
src/awkward/_v2/operations/ak_from_cupy.py	50.00% <ø> (ø)
src/awkward/_v2/operations/ak_from_jax.py	50.00% <ø> (ø)
src/awkward/_v2/operations/ak_from_json_new.py	0.00% <0.00%> (ø)
src/awkward/_v2/operations/ak_from_numpy.py	100.00% <ø> (ø)
... and 109 more

[jpivarski]

There were fewer of those than I thought there was going to be.