Fix deserialization of root key id

[preuss-adam]

• Fix missing root key id in construction of SerializedBiscuit.
• Use root key id directly from SerializedBiscuit to avoid losing it when copying objects.

[Korbik]

Looks good to me.

Is it necessary to be integrated in a fix release ? Because the next release will be a major one with some break with the API.

For a quick fix, we may need to create a new branch to release the fix and rebase the branch from the 4.0.1 tag. Let me know what you prefer.

[preuss-adam]

Thanks for review. I think we just merge this now and have new release when it is appropriate.

I'm more interested in a release because of the fix to check the result of verification in: #108