Create a doughnut-registry runtime module.

[jordy25519]

It should be a simple key-value store of hash(doughnut)-> doughnut.
It requires a public call which takes a doughnut (<T as system::Trait>::Doughnut) and stores it keyed against it's hash.

This will serve as a base for doughnut storage prototyping and will allow us to try periodisation, revocation, and the like. A more sophisticated storage solution can be designed in parallel as we become more familiar with requirements.