Introduce `Friend` extending `User`
Introduce `User` acting as the superclass of `Friend` and `Member`

Add extensions `Member.asFriend`, etc.
Remove specialized `Contact.hashCode` and `Contact.equals`, inherit from `kotlin.Any`
Specify `toString` behavior in abstract `Contact` classes, instead of in implementation classes.

Add virtual member `Contact.sendMessage(String)`

Migrate the project from using `QQ` to `Friend`

Remove multiplatform Contact class structure, keep them common.

Remove extension `Long.at` in `GroupMessage`, `QQ.at` in `ContactMessage`.
Add `At.asMember()` in `GroupMessage`.

Make `calculateGroupUinByGroupCode`, `calculateGroupCodeByGroupUin` static on JVM.