Joining two multi-width chars should not involve a space

[lichray]

From email

last char first char behavior

---

multi-width multi-width nothing ins'ed
multi-width single-width 1 spc ins'ed
single-width multi-width 1 spc ins'ed
single-width single-width original

http://mail-index.netbsd.org/tech-userlevel/2017/11/06/msg010940.html

It's very unlikely we can find a language with
visually wide characters but still spaced with
single width spaces.