support struct sort about pdqsort

[zhangyunhao116]

Happy to see TiDB plan to use the new implementation!
The generic version of pdqsort is located in https://github.com/golang/exp/tree/master/slices for now, and it will be moved to the std in the future. The problem is that the APIs in the slices may be changed at any time. A possible solution is to copy the related code into the TiDB project, once the slices is merged into std, we can easily change the code.
For this repo, I'm still not sure whether it needs to be maintained, since it has already been merged into the Go runtime.

[hawkingrei]

Thank you for your suggestion. I have started to use https://github.com/golang/exp/tree/master/slices to implement it.