Add caching to Hamt and update testing

[austinabell]

Summary of changes
Changes introduced in this pull request:

• Adds cache to Hamt, similar to one added with Amt refactor and interop tests #716
• Removes murmur3 support
  • No need for it anymore
• Update serialization tests for sha hashing, and include blockstore vectors (testing against austinabell/go-hamt-ipld@5ee4d42)
  • Our reads and writes are a LOT lower than the go impl, but we will have to match with Add redundant writes to Amt and Hamt to match go impl #729 (splitting up to make it easier to revert if needed in future)

Reference issue to close (if applicable)

Closes #719

Other information and links