Use safe concatenation.

[riga]

In awkward, arrays that are the result of a mask being applied to a larger array are not copies. They appear to have a shorter length (in the desired dimension) but internally, they still seem to reference the original, unmodified data (as can be seen when printing the number of bytes via array.nbytes, which still reflects the original value).

Using these arrays in ak.concatenate operations results in the original data being concatenated as well, which is undesired in most use cases in cf as it needlessly leads to excessive memory footprints.

Wrapping these masked arrays by ak.Array or ak.copy does not apply the initial mask, but just returns a view or full copy of the array (which, again, still contains a reference to the unmodified data). Only ak.to_packed fully resolves masks (in addition to yet un-materialized columns, which it was originally intended for). Fortunately, it does not copy arrays that are already fully materialized.

This PR adds a util ak_concatenate_safe that does the same as ak.concatenate, but wraps each input array by ak.to_packed, avoiding all the aforementioned issues.

[mafrahm]

LGTM, just one question for my own understanding