[spell-check] Rewrite/reformat for ease of understanding…

[savetheclocktower]

…with a couple of bug fixes along the way.

Part of this is to at least give folks who suffer from #1287 something to test out in the unlikely event that this fixes it.

But the spell-check package is full of barely decaffeinated code that was begging for simplification. I'm also quite certain that I caught one transcription bug when attaching subscriptions, and maybe that's the cause of the trouble?

But at any rate, the tests still pass and the code is more readable.

[savetheclocktower]

(I mean, the tests passed on my machine, so…)