Obsolete string-to-multibyte hard to replace

[Stefan Monnier <monnier@iro.umontreal.ca>]

I was looking at ruler-mode.el's bytecomp warning about string-to-multibyte
and it seems like there is no good way to write cleaner code here:

   (let (...
         (ruler
          (propertize
           (string-to-multibyte
	    (make-string w ruler-mode-basic-graduation-char))
            ...))
         ...)
    [...]
          (aset ruler k (aref c (setq m (1- m))))

Here, the problem is as follows: make-string returns a unibyte string if
the initial char is ASCII and a multibyte string otherwise.
Seems innucuous, but it means that if the initial char
(ruler-mode-basic-graduation-char above) is ASCII, you can't later
insert a multibyte char into that string with `aset`.

Replacing string-to-multibyte with decode-coding-string is clearly wrong
in the above case (I mean it'll work, but it's even more of a hack than
using string-to-multibyte).

While investigating it, I noticed that my local Emacs hacks include
a change of make-string so it always returns a multibyte string
(I think I made this change, because an all-ASCII multibyte string is
a more precise information than an all-ASCII unibyte string: with
a multibyte string, the fact that `x->size == x->size_byte` immediately
tells us this is an ASCII-only string whereas with a unibyte string we
can't know if that string is ASCII-only without checking each and every
byte).

I think it'd make sense to change make-string so it always returns
a multibyte string, and maybe to also introduce a new make-unibyte-string.


        Stefan