ENH: Provide functions to convert the axes of object arrays to and from lists

[psimaj]

Closes #14478.
This pull request implements functionalities asked for in #14478.
I decided to implement the function converting arrays to arrays of lists in a way that allows choosing the axis to convert, as if we always converted the last (or next-to-last, depending how we look at it) axis, then those functions wouldn't be inverses of eachother; for example, converting
array([list([[1, 2], [3, 4]])])
with array_of_lists_to_array results in
array([[[1, 2], [3, 4]]])
but converting the last axis of the above array into lists would yield
array([[list([1, 2]), list([3, 4])]])
which is different to what we started with.
I would greatly appreciate opinions on the above change and regarding the names of the functions and their placement in the project structure (currently placed in lib/shape_base.py).

[psimaj]

@eric-wieser Could you please take a look at and review this PR? I see that some tests aren't passing, but I am not sure what exactly is wrong (locally everything passed) and it's my first PR, so I'd appreciate a little guidance.

[charris]

Should probably run this past the numpy mailing list.

[InessaPawson]

@psimaj Thank you for sharing your ideas! If you'd like to move this PR forward, I recommend asking for feedback on the NumPy mailing list: https://mail.python.org/archives/list/numpy-discussion@python.org/.

[mattip]

I think we should close this, as neither the issue nor the PR gained much traction.

[rossbar]

I'll go ahead and close this - if there's strong desire to push this idea forward, following up with the mailing list seems like a good place to start.