SectorSet: AMT design spike

[ZenGround0]

Description

We need to solidify the design of the A[rray]M[apped]T[rie] structure proposed as the implementation for sector sets

We should write this as a subpackage of actor. This issue is done when we have a plan for all of the software changes needed to make this AMT in software and spec it out in prose.

Acceptance criteria

Build a prototype AMT to implement SectorSets. Likely file several issues after this spike for writing/factoring into something more robust. Don't forget to track work for writing up the spec for this.

Risks + pitfalls

Don't rush in without understanding what's going on BUT
Don't spend too much time reading about it before trying to build it

Where to begin

Read through the design doc to get the background and relevant links.

go-hamt-ipld spec and code, the Bagwell HAMT paper and the sectorset spec issue (links in doc) are all going to be useful.

[ZenGround0]

This currently exists in this spec PR. It is low priority for me to get this spec PR through to completion but I'd like to get to it within a few weeks. Word on the street is that @whyrusleeping plus some others might be leading the implementation charge on this one.

[anorth]

@ZenGround0 I think we can close this now. The AMT still needs to be implemented, but that's a different issue, and the work may happen outside go-filecoin.