Fix handling of double-column characters #749

This is probably an issue with curtsies, but until I'm sure I want to report it here first:

Steps to reproduce: Enter something like '間'.isalnum(), then move the cursor back (left arrow key) such that a is the next character, and insert an x.
Expected behavior: Visually it looks like we're about to insert an x between is and alnum. So I would expect the line to say isxalnum.
Actual behavior: It now says '間'.isaxlnum(), with the next character seemingly being the x.

Try this out with y instead of 間 to see the expected behavior.

I may have broken this in the patch to #703, I'm not sure.

The pun ("間" meaning "between") is entirely coincidental, but I like it, and I'm keeping it.