字面值字符串的拼接

仅仅是个编译器的小trick。实用价值不大。
NSString *s1 = @"a" @"b" @"c"; => @"abc";
NSString *s2 = @"a" "b" @"c" "d"; => @"abcd";
char *cs = "a" "b" "c"; => "abc";
注意：字面String的空格不是必须的，可以去掉