Implement multimap

[austinabell]

Summary of changes
Changes introduced in this pull request:

• It's a hamt where the values are amts
  • Used for indexing addresses and epochs and saving an ordered set of values
• Refactors Amt to get it closer in line with the Hamt for more consistent usage

Note: the array value type must be inferred when retrieving since only a cid is saved at the nodes of the Hamt, so still have to be careful on usage to make sure the Amts get deserialized correctly

Reference issue to close (if applicable)

Closes #280

Other information and links