Checks if a string is valid utf8
"hallo".is_utf8? # returns true or false
"hallo".is_utf8_raw # returns 0 if true or the position where a offending char is found
#include <mruby/string_is_utf8.h>
mrb_value str = mrb_str_new_lit(mrb, "hallo");
mrb_str_is_utf8(str); // returns TRUE or FALSE
mrb_str_is_utf8_raw(str); // returns 0 if true or the position where a offending char is found