Rework types in power actor cron event registration

[ZenGround0]

Right now the miner actor sends arbitrary bytes to the power actor to register events. This is likely unnecessarily unconstrained as the current set of cron events are fully captured by an integer type.

We should investigate changing the cron enrollment system over to using more constrained types. If we do this it will require migrating the cron event queue.

[laudiacay]

I put some work in fix/1490. Blocked in "why isn't my CBOR gen working" land, once again... I don't know why this is so confusing to me