Clean up assoc_array library

Nils Ollrogge reported a bug in the assoc_array library and had a patch
to fix it too, but that code was too obscure for me to follow, so I did
what I preach: use modern LLM tools to help make sense of it all.

So this is an LLM-assisted branch which makes the code more readable by
splitting up some of the larger and hard-to-read functions.  The code
was then run through a test harness in user space, which not only
confirmed the bug Nils reported, but found three other bugs too - all
going back to the very first commit adding this library back in 2013.

v2: David Howells asked for a fix-up of a couple of comments and rename
assoc_array_segment_leaves() to assoc_array_fill_segment_cache(), and
added his Tested-by: and Reviewed-by:
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