Make `DiffableDataSourceSnapshot` struct

[kishikawakatsumi]

In the latest Xcode 11 (beta 6), DiffableDataSourceSnapshot class changed to a struct. This PR adopt the API to the latest UIKit's API.

Checklist

• [x] All tests are passed.
• [ ] Added tests.
• [ ] Documented the code using Xcode markup.
• [x] Searched existing pull requests for ensure not duplicated.

Description

Motivation and Context

If we can write let snapshot as before, we can't copy and paste as it is when we migrate to the iOS 13 standard API in the future. Because of let snapshot almost causes compilation error on in Cocoa's API.

Impact on Existing Code

In most cases let snapshot is now a compilation error. Users should rewrite it to var snapshot.

[ra1028]

Great catch up @kishikawakatsumi ! Thanks 👍