字符串的全操作，重载+、=，重载输入输出流，比较串长，清空串，曲取子串，插入函数，查找函数，正序查找，倒叙查找。-All string operations, overloaded+, =, overloading input and output streams, more string length, empty string, substring take music, insert function, search function, find the positive sequence