Add lower-level methods for concatenation of double-nested columns

[hombit]

Feature request

We currently don't support multiple nesting, but packing an already nested column may lead to double-nesting. It would be nice if we could provide a lower-level interface to convert these double-nested columns back to single-nested ones.

One possible aggregation method would be flattening the inner nested values, which is essentially the same as concatenating the double-nested frames into a single frame.

This feature request replaces #201, which provided additional motivation for this feature but proposed a higher-level interface.

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