New sub-package `github.com/huandu/go-clone/generic` with generic APIs

Package clone supports generic now!

• For users who want to opt-in Go generic syntax for cleaner APIs, new package github.com/huandu/go-clone/generic is your best choice. To import this package, Go 1.18+ is required.
• For users who are not ready for Go 1.18+, you can continue to stick on github.com/huandu/go-clone.

All features of these two packages will be the same in the future.