The Canonical type is generally constructed with the threshold, needed for the quorum computation, in scope. To make it easier to use and encapsulate its functionality, it now holds the threshold as an additional field.

Changes for the Canonical type:

• Reorder arguments so that delegates and threshold are grouped after refname
• Add Canonical::is_empty to detect when Canonical was constructed with no tips
• Consume Canonical when calling Canonical::quorum to indicate that it should be the final step