add BlockRecord

[arvidn]

This PR is best reviewed one commit at a time.

This adds the BlockRecord type, defined in python here.

The main challenge in this type is implementing sp_sub_slot_total_iters(), ip_sub_slot_total_iters(), sp_iters(), ip_iters(), sp_total_iters(). The underlying functions used by these are implemented in python.

This patch calls back into python to run these functions. It's not elegant or efficient, but it's a stop-gap until we have ported these functions to rust.

In order to ensure this works, a fuzzer constructs an arbitrary BlockRecord object and calls all these functions on both the rust and python version, ensuring the results are identical, including failures.

[coveralls-official]

Pull Request Test Coverage Report for Build 7402035226

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 85.897%

---

Totals
Change from base Build 7401016788:	-0.1%
Covered Lines:	11128
Relevant Lines:	12955

---

💛 - Coveralls

[Rigidity]

Looks good for now, would definitely like to remove any calls back into Python at some point