Add record-preserving for behaviour to ak.flatten

[agoose77]

As discussed in the user docs, there is currently no straightforward way for a user to completely flatten an array without losing its record structure, e.g.

>>> array = ak.Array([{'x': 1, 'y': 'hi'}])
>>> ak.flatten(array, axis=None)
<Array [1, 104, 105] type='3 * int64'>

@jpivarski mentioned on Gitter the idea of adding a new axis="records" case to ak.flatten which preserves (nested) record fields.

[jpivarski]

@agoose77 I changed my mind. This does not have to be a next-release issue. We can leave it for the far future.