Introduce the `rules` module beneath `canonical`. The implementation follows
the Canonical References RIP[^0].
A `Rule` defines the DIDs and threshold required to get the canonical tip
for a reference (or set of references). Since a `Rule` has some specific
validation rules, there can be two versions of a `Rule`. The `RawRule` is one
that can be easily constructed, serialized, and deserialized. It must then be
validated into a `ValidRule`, which validates any constraints, and can be used
in the computing of a canonical tip.
For convenience, a set of `ValidRule`s is wrapped into the `Rules` type, which
also has a further constraint that each rule refers to a specific `refspec`.
[0]: 1d1ce874f7
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.
Fintan Halpenny
Lorenz Leutgeb