std::unicode::icu: add libicu character category functions

[kud1ing]

Add bindings to libicu's (http://icu-project.org/apiref/icu4c/uchar_8h.html) functions:

• UBool u_isdigit (UChar32 c)
• UBool u_islower (UChar32 c)
• UBool u_isspace (UChar32 c)
• UBool u_isupper (UChar32 c)
• UChar32 u_tolower (UChar32 c)
• UChar32 u_toupper (UChar32 c)

[kud1ing]

Fixed by #1377