Venus: `Prover#VerifyPoST` should be passed the challenge seed used to generate the proof-of-spacetime

[laser]

Description

In order to verify a proof-of-spacetime, you need the inputs used when generating a proof-of-spacetime. The existing API design doesn't permit callers of the Prover#VerifyPoST method to provide the challenge seed used as input to Prover#GeneratePoST, which is a bug.

Acceptance criteria

Risks + pitfalls

Where to begin

[laser]

Things remaining:

1. Shannon needs to merge her changes to go-filecoin master. Those changes include a call to verify_post with the challenge bytes, but commented out.
2. Once the aforementioned thing happens, Sidney will merge the rust-proofs changes to master.
3. Once the aforementioned thing happens, Sidney will update the go-filecoin submodule SHA to point to the latest and greatest rust-proofs, and will remove the comments.

[shannonwells]

As part of this there needs to be some refactoring (in a separate branch); these changes break a ton of tests, for one. Just a heads-up.

[laser]

Remaining issues:

• The proof-of-spacetime generated by a storage miner M while "storage mining" (term borrowed from @ZenGround0) needs to generate a challenge seed using the block(s) at height N, where N equals the block height of at the beginning of M's current proving period.

[laser]

~This work is currently blocked on this go-filecoin issue.~

This work is currently blocked on this go-filecoin issue, which will allow chain to be sampled for randomness outside of the state machine.