继续完善MyString，但是un.cpp编译出现奇怪错误
搞不懂
Revert "搞不懂"

This reverts commit de78aa52f0b1194734452f3e6f5cd682fc156775.
Revert "继续完善MyString，但是un.cpp编译出现奇怪错误"

This reverts commit cc38586c4ec4874493c1988172ec6120da6b7f02.
MyStringUnitTest报错，提示需要合适的ToString()问题解决