feat: delegation.toJSON

[Gozala]

This supersedes #173 and Fixes #172. It targets #185 as it relies on changes there.

In the other PR @alanshaw had this suggestion

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

And this PR still does not add fromJSON because, even though it will serialize the whole chain into a JSON which could be then de-serialized into compatible chain there is still a problem with the fact that fromJSON would not know if inline proofs do match claimed CIDs. This means that motivated attacker could use this an a attack vector.

In other words code that imports delegation should also validate hashes which is async operation and there for does not fit into a fromJSON signature.

That said I think it still useful to have toJSON method for debugging purposes, which is what this PR adds.

[gobengo]

awesome

[vasco-santos]

lgtm thanks @Gozala