[fix] Unicode::utf8Prev() does not iterate correctly
[enhance] more utf8 iteration functions
[fix] win32 : isalnum assert fails when ch < 0
[fix] script: %c specifier now can handle unicode value
[change] script: Make utf8 iteration as default for string. Resolves #12
- foreach on string iterates by unicode-index
- string.len() returns unicode length
- utf_*() functions are renamed without utf_ (as default)
- old ascii-based functions are renamed to ascii_*()
- all string functions support negative indices