Amt refactor and interop tests

[austinabell]

Summary of changes
Changes introduced in this pull request:

• Fixes bug with Amt delete. In the case that a delete happens on an index that doesn't exist, the function would return without replacing the dirty cached node
• Adds cache for cid resolution
  • Allows skipping bs resolution of Cid link if it's cached to avoid duplication (necessary for interop)
  • Removed Clone requirement by allowing a ref to the cache to be returned for loaded subnodes
• Adds tests for serialization and blockstore stats
  • Cids are all the same, just made them string format to remove need for hex encoding
  • Our impl performs less writes on expansion, because go implementation flushes unnecessarily (will follow up but added TODOs)
• Adds TrackingBlockStore which allows stats for reads and writes to be used to compare against Lotus (gas usage is based on this, so will be useful for Hamt as well which is why it's in a shared location)

Reference issue to close (if applicable)

Closes

Other information and links

[ec2]

No biggie, but shouldnt you .enumerate instead?

[austinabell]

nah, because I want a u64 not a usize (and don't want to cast)