feat: implement toJSON on delegations

[Gozala]

Fixes #172

[alanshaw]

If we get a fromJSON we can remove some code for storing delegations in the access Conf store.

[Gozala]

If we get a fromJSON we can remove some code for storing delegations in the access Conf store.

I don't think fromJSON is visible because delegation formats the block not the chain, which is to say that fromJSON(toJSON(delegation)) is going to be lossy because all of the proofs will be turned into links, which is why fromJSON was never part of the API.

I've created #174 to discuss this further, I don't think we should block this

[alanshaw]

It's very unexpected for toJSON to be lossy. I wouldn't have suggested fromJSON if I'd known that was the case.

It either needs to be commented heavily or renamed. I'm not even sure if commenting is enough since toJSON is called transparently by JSON.stringify.

[vasco-santos]

Let's discuss this further once we get first prod release in place