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This is done by returning an `io::Error` with
`ErrorKind::UnexpectedEof`. This error is only returned if the buffer
was empty when ctrl-d was pressed.

This is necessary to support proper undoing in insert mode.

Prior to this patch, typing some text, moving to the start of the line,
then deleting until the end of the line and attempting to undo will
cause a panic. This is because the truncate() operation did not update
the undo list. This has been corrected.

- properly handle zero/single/double-width characters
- don't count CSI escape codes as characters in the prompt