Fix #3741, simplifying things in the process
The problem in #3741 was that we had confused column numbers with byte offsets, which fails in the case of UTF-8 (amongst other things). Fortunately we're tracking correct column offsets now, so we didn't have to make a calculation based on a byte offset. I got rid of two fields from the PState (last_line_len and last_offs).and one field from the AI (alex input) constructor.
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