Miner state

[austinabell]

Summary of changes
Changes introduced in this pull request:

• Implements miner state
  • Some epochs can be optional which explains the OptionalEpoch type. They need to be serialized as -1 for undefined. It is possible that undefined epochs are intended for other types other than this, but I think it is contained to this. Otherwise we will find out later
• Updates for_each function signatures in Hamt/Amt to be more consistent, cleanly used, and not clone when not necessary

One todo blocked on #308 which I will do after that comes in

Reference issue to close (if applicable)

Closes #321

Other information and links