EIP2935 - Save historical block hashes in state

# Simple Summary

Store historical block hashes in a contract, and modify the BLOCKHASH (0x40) opcode to read this contract.

# Motivation

There is increasingly a desire to remove the need for most clients to store history older than some relatively short duration (often between 1 week and 1 year) to save disk space. This requires some form of layer-2 network to help clients access historical information. These protocols can be made much simpler if blocks contained a quick Merkle path to historical blocks.

Additional secondary motivations include:

The protocol can be used to make more secure efficient light clients with flyclient-like technology (while the "optimal" flyclient protocol is fairly complex, large security gains over the status quo (trusted "canonical hash trees") can be made cheaply)
Improving cleanness of the protocol, as the BLOCKHASH opcode would then access state and not history.

# Specification

Parameter	Value
`FORK_BLKNUM`	TBD
`HISTORY_STORAGE_ADDRESS`	`0xfffffffffffffffffffffffffffffffffffffffe`

At the start of processing any block where block.number > FORK_BLKNUM (ie. before processing any transactions), run sstore(HISTORY_STORAGE_ADDRESS, block.number - 1, block.prevhash).

When block.number > FORK_BLKNUM + 256, change the logic of the BLOCKHASH opcode as follows: if FORK_BLKNUM <= arg < block.number, return sload(HISTORY_STORAGE_ADDRESS, arg). Otherwise return 0.

# Rationale

Very similar ideas were proposed before in EIP-98 and EIP-210. This EIP is a simplification, removing two sources of needless complexity:

Having a tree-like structure with multiple layers as opposed to a single list
Writing the EIP in EVM code

The former was intended to save space. Since then, however, storage usage has increased massively, to the point where even eg. 5 million new storage slots are fairly negligible compared to existing usage. The latter was intended as a first step toward "writing the Ethereum protocol in EVM" as much as possible, but this goal has since been de-facto abandoned.

# Backwards Compatibility

The range of BLOCKHASH is increased by this opcode, but behavior within the previous 256-block range remains unchanged.

# Test Cases

TBD

# Implementation

TBD

# Security Considerations

Adding ~2.5 million storage slots per year bloats the state somewhat (but not much relative to the hundreds of millions of existing state objects). However, this EIP is not intended to be permanent; when eth1 is merged into eth2, the BLOCKHASH opcode would likely be repurposed to use eth2's built-in history accumulator structure (see phase 0 spec (opens new window)).