feat(mls group): make MlsGroup cloneable

[SimonThormeyer]

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[coriolinus]

I think we don't actually want a Clone derive for MlsGroup, even though we have a use case for it.

Recommendation: derive Clone for private/crate-private items here; that's harmless. But where there's a chance that an external user has visibility of a struct, manually implement fn fork(&self) -> Self, with some documentation to the effect that this is like Clone, but you really should only do this in circumstances where you know that you want to fork the state.

[coriolinus]

See above, meant to request changes there.