用法：
编译时添加引用/r:d:/path/MyStringDll1.2.dll
代码示例：
using Falls27.Utils;
MyString ms=new MyString(); //初始化
Console.WriteLine(ms.IndexOf("hello world","l",5)); //输出9
Console.WriteLine(ms.Substring("hello world",1,6)); //输出“ello w"
Console.WriteLine(ms.ReplaceLast("hello world","l","*")); //输出”hello wor*d"
Console.WriteLine(ms.ReplaceAll("hello world","l","*")); //输出”he**o wor*d"
ArrayList list=ms.GetPosList("hello world","l");
foreach(int pos in list){
Console.WriteLine("" + pos);
} //输出2，3，9
