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
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.
Lorenz Leutgeb
Jakob Kirsch
Fintan Halpenny