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.