ISRI stemmer: Corrections to logic (and many style fixes)

There are two commits here. The first is a simple bug fix that resulted in the literal string "\u0627" being put into stemmed Arabic forms instead of the unicode character \u0627. The second is a more substantive change, fixing two fundamentally flawed points in the logic of the code, and cleaning up a lot of things stylistically.

Please do note, as I say in the latter commit, that I don't speak any Arabic, and I didn't compare this to the original paper laying out the ISRI stemmer. So while I left most of the substance of the code alone, I'm not trying to implicitly endorse it as correct; I was only changing things that I knew couldn't be correct.