Export remove_structure and prettyprint for dask-awkward

[agoose77]

Description of new feature

Dask-awkward currently imports this private name.

[jpivarski]

Here:

https://github.com/dask-contrib/dask-awkward/blob/816eebaba6765e390c483a212d3fd3b37ec65d97/src/dask_awkward/lib/core.py#L19

The Ragged library almost had to do this, but I found a work-around. However, Ragged does use _prettyprint:

https://github.com/jpivarski/ragged/blob/444aa0f7016688ce0e9a8fa828c73cc7ac2a3210/src/ragged/_spec_array_object.py#L239-L267

Both of these should be exported as L2 "semipublic". They shouldn't be ak.* functions, but hidden in some nested submodule, though without an underscore.

This issue mentions both of them because it's as easy to do both as it is to do either one of them. We just need to decide on what their API paths will be.