[bug-libunistring] please, update int to size_t

[Ощепков Константин]

Some functions in libunistring return "int" as string length. See 
https://www.gnu.org/software/libunistring/manual/libunistring.html:

 int u8_width(const uint8_t *s, size_t n, const char *encoding);

It takes string and "size_t" (64-bit) as input and return "int" (32-bit) as 
output.
See also u16_width(...), u32_width(...), uc_width(..), u8_strwidth(...), 
u16_strwidth(...), u32_strwidth(...).
I can't count my 3 Gb string. Please, make them return "size_t".
Thank you very much!