Most of this state is used to manage interactions with other state,
and encapsulation allows us to hide data structures and access behind
self-explanatory function names.
The interface is still much wider than I'd like, but it can be improved in
future work.
There is one small behavior change in here: in the `follows` edge case
demonstrated by I3bdf26fd14, we prepend if the splice point cannot be
used because it has a non-numeric key. I believe this was the original
intention of the logic, and is how the numeric case behaves. I've verified
that when array_splice throws a warning about non-numeric key, it fails to
add anything to the original array, so the broken follows ref disappeared.
Bug: T237241
Change-Id: I091a0b71ee9aa78e841c2e328018e886a7217715
