fip-0100: sectoronchaininfo, deadline and expirationset type changes

[rvagg]

Needs one of whyrusleeping/cbor-gen#111 or whyrusleeping/cbor-gen#112 first; and this isn't complete, I have ExpirationSet and Deadline serialization tests to add to match https://github.com/filecoin-project/builtin-actors/blob/481e18161e0af9a9a076327569c4e90fccf23fb6/actors/miner/tests/types_test.rs

[rvagg]

@Stebalien this is good to go but with the big.Int{} uninitialised. For now it's just documented where it matters that you need to take care touching that value. I think in practice that the only one that will be used (outside of invariants checks) is SectorOnChainInfo.DailyFee but it's going to be used in serialized form for the API and the serialisers already treat nil as zero so I think that should be fine. It might matter if miner software starts touching it though.

[Stebalien]

I'd say merge this as-is for now while we figure out the nil/zero stuff. LGTM.