mmap Union of bits types

[tpapp]

Now that arrays with Union of bitstypes elements have an optimized layout, it would be useful if one could mmap them. Alternatively, exporting the API that would allow the user to mmap the constituent flag and content vectors, then "assemble" the array, would also be as solution.

Cf

1. a related issue on exporting the API: API for directly accessing the selector bytes of an isbits Union Array #24057
2. discussion (I am opening this as a reminder, as suggested)
3. I would use it in this package

[Keno]

The in memory representation of union bits types is not (and is not supposed to be) stable. However, you can achieve the same performance by an array view type of the appropriate element that uses the same logic (implemented in julia) and that'll keep working if we decide to change the representation.

[tpapp]

Revisiting this issue, I am wondering if the representation has stabilized yet. It would be nice to mmap eg Vector{Union{Missing,Int}} and similar.