Implement Turboproofs

[zah]

Nimbus should become a highly efficient Eth1 stateless client. To execute a block, a stateless client needs to obtain only a small portion of the state that is being referenced by the transactions in the block. The needed data can be downloaded on demand and its integrity can be verified through merkle proofs. We call this data "the witness" for the block.

The Turboproofs scheme is an efficient method for compressing the block witness that was proposed by Alexey Akhunov. Existing implementations exist in Rust and TypeScript.

Goals of this bounty:

• Implement a module with low-level routines for working with turbo-proofs.
• Collect test vectors from existing implementations and implement a test suite.
• Add a routine that can generate a turbo-proof by executing a block and keeping track of the referenced state trie data.
• Add a routine that can populate a trie database (e.g. created with newMemoryDB) with the contents of a turbo-proof.

[zah]

A new specification for the block witnesses is being published here:
https://github.com/ethereum/stateless-ethereum-specs/pull/1/files

[zah]

@jangko, my understanding is that we already have everything needed to close this?

[jangko]

that's right, we can close this one.