Feedback on Data Structures Section MEGA Issue

This issue has a bunch of items. Feel free to check them off as they get completed or tracked in other issues.
Block:
- [x] The Signature field is missing from the block data structure and should be discussed
- [x] Separate from the Ticket the block must also include an array of time delay proofs (PoST now, moving to VDF soon). One proof per null block
- [x] The Parent Weight fields must move to a single parent weight field. The data format and some reasoning about why it is good enough is captured in [this recent change to go-filecoin](https://github.com/filecoin-project/go-filecoin/pull/1523).
- [x] We need to change both Messages and MessageReceipts to a `MessageTreeRoot` type
- [x] I don't see SortedCidSet defined in this section (maybe the cid spec?) it should be clear where to read about this thing.

Message:
- [x] From TODO -- discuss how method parameters get encoded
- [x] Serialization -- from @phritz we should vet our serialization process for efficiency, potentially improve it and reflect that work here. (Same comment about actor serialization which is the same.

State Tree:
- [x] There appears to be insufficient detail explaining how to use a HAMT to implement a State Tree

HAMT:
- [x] From TODO -- we need to spec this thing out, especially since this isn't really a HAMT

MessageTree:
- [ ] We need to create a spec for this thing. Likely this will look a lot like state tree and we will implement it using the same HAMT.

Thanks!

Start with the unchecked MessageTree item and compare its stated relationship to the State Tree and shared HAMT implementation. Define the scope and required content of the MessageTree specification, then track completion in this issue or a separate issue.

Written by the indexing model from the issue text.