format: Count prefix and suffix width in terms of display columns.

Until now, the prefixes and suffixes for custom currency formats
(CCA, etc.) have been considered to occupy one display column per
byte.  This is fine for prefixes and suffixes like "$" or "%", but
falls down badly with U+00A5 (¥) or U+20AC (€), which occupy two
or three bytes, respectively, in UTF-8, while occupying only a
single display column.

This commit fixes the problem.  It doesn't add a test yet because
there are still some higher-level issues, but that will come in
a later commit when those remaining issues are resolved.