core: improve case convert and insensitive char comparisons (closes #258)

core: improve case convert and insensitive char comparisons (closes #258)

All lowercase letters are now properly converted to uppercase letters (and vice
versa), via functions `towupper` and `towlower`.

Functions `string_tolower`, `string_toupper` and `utf8_charcasecmp` have been
optimized to be faster when there are ASCII chars (< 128); functions are about
25-40% faster with mixed chars (both ASCII and multi-bytes).

Function `utf8_wide_char` has been removed, `utf8_char_int` can be used
instead.