bug#43866: 26.3; italian postfix additions

[Juri Linkov]

> The implementation seems to rely on a file in the /usr/include tree
> that might not be there.  This is a significant disadvantage, IMO.  It
> means that, unlike all other similar facilities in Emacs, this one is
> not self-contained.
>
> Is it possible to lift this limitation?

Yes, this is easy to do.  But I have one problem:
/usr/share/X11/locale/en_US.UTF-8/Compose contains 83 lines
where a key sequence maps to 2 characters, not to 1 character, e.g.

  <Multi_key> <acute> <Cyrillic_u> : "у́" # CYRILLIC SMALL LETTER U WITH 
COMBINING ACUTE ACCENT

where "у́" is 2 characters: CYRILLIC SMALL LETTER U and COMBINING ACUTE ACCENT.

iso-transl.el maps a key sequence to a single character only using

  (define-key map (apply 'vector '(?' ?у)) (vector ?у))

I don't know how to map a key sequence to 2 characters.
When trying to map to 2 characters ?у and ?́ :

  (define-key map (apply 'vector '(?' ?у)) (vector ?у ?́ ))

typing 'y inserts only the last character ?́ , not both ?у and ?́ .