refactor: remove to_arraylib

[agoose77]

This PR

• Removes ak._util.to_arraylib and
• Deprecates NumpyArray.to_numpy(allow_missing) in favour of
• Adds NumpyArray.to_backend_array(allow_missing, backend)
• Deprecates support of UnionArray in to_backend_array().

to_arraylib() duplicates much of to_numpy()'s logic, and we also don't need to enshrine NumPy as more special than the general backend-agnos

[codecov]

Codecov Report

Merging #2128 (69465ea) into main (aa202ee) will increase coverage by 0.12%.
The diff coverage is 82.20%.

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/_connect/rdataframe/from_rdataframe.py	0.00% <ø> (ø)
src/awkward/_util.py	76.53% <ø> (+2.78%)	⬆️
src/awkward/operations/ak_to_cupy.py	33.33% <16.66%> (ø)
src/awkward/_nplikes.py	64.46% <50.00%> (-0.06%)	⬇️
src/awkward/contents/bitmaskedarray.py	68.02% <66.66%> (ø)
src/awkward/contents/bytemaskedarray.py	88.80% <66.66%> (ø)
src/awkward/contents/indexedarray.py	77.66% <66.66%> (ø)
src/awkward/contents/content.py	72.94% <71.42%> (-0.19%)	⬇️
src/awkward/contents/unmaskedarray.py	74.04% <75.00%> (+0.85%)	⬆️
src/awkward/contents/unionarray.py	84.98% <80.00%> (+0.59%)	⬆️
... and 15 more

[jpivarski]

It's great that the implementations can be unified, but there needs to be a high-level public ak.to_numpy, if only in name. There will be a lot of users who don't even know (or care) that Awkward runs on non-NumPy backends, and this will make them unlikely to find a function named ak.to_backend_array.

Nevermind: I see that you're leaving the L1 interface ak.to_numpy and just removing the L2 Content.to_numpy interface.

Currently, only NumPy has a way to use allow_missing=True; CuPy, etc. don't have a ma submodule or a MaskedArray equivalent. But in principle, they might someday get such an interface, and we can rename it through nplike if it comes in with a different name.

It's good to have the deprecation message specify version 2.2.0 like this, to always target two minor versions ahead. When this is merged, please add it directly to

https://github.com/scikit-hep/awkward/wiki#api-breaking-changes-after-20

to make sure that this list doesn't get out of date. The warning message is for developers who stumble onto this deprecation by calling Content.to_numpy, but the wiki is for a more global view: "What are all of the things that will be deprecated in version X?"