Fix math alignement ('&') when math contains non-ascii characters

[arnaudgolfouse]

So I noticed that alignment formatting in math mode sometimes gives strange result:

$
x & = y \
α & β
$
// becomes
$
  x  & = y \
  α & = β
$

It was due to usages of s.len(), which I replaced with s.chars().count().

Note that this does not attempt to handle unicode characters with weird widths, like

ＡＢＣ
ABC

(Yes, it's real: those are U+FF21, U+FF22 and U+FF23, known as "fullwidth" ascii characters)

[astrale-sharp]

Amazing thanks :D