- unicode_nfkc_casefold(+In, -Out) is det
- Equivalent to unicode_nfkc/2 followed by unicode_casefold/2 done
in a single pass. This is the normalisation form recommended by
UAX#31 for caseless identifier matching. For example German
'Strasse' written with U+00DF (LATIN SMALL LETTER SHARP S)
maps to 'strasse', and U+FB03 (LATIN SMALL LIGATURE FFI) maps
to 'ffi'.