Eth 2.0 Phase 2 with "precompiles" proposal

[axic]

The execution engine part is mostly found here: https://notes.ethereum.org/s/HylpjAWsE#Shard-processing

Official discussion thread: https://ethresear.ch/t/a-layer-1-minimizing-phase-2-state-execution-proposal/5397

We should investigate the feasibility of this proposal in ewasm and provide feedback on the ethresearch forum.

[axic]

A translation of the proposal into (non-comprehensive) Ewasm design follows:

1. Entry point is main(), receives no arguments
2. The following EEI methods are defined:

• getDataLength() -> i32
• copyData(offset: i32ptr, length: i32, memoryOffset: i32ptr), where memoryOffset is the memory area of the current execution to copy data to
• setStorage(key: i32ptr, valuePtr: i32ptr, valueLength: i32)
• getStorageSize(key: i32ptr) -> i32
• getStorage(key: i32ptr, offset: i32, length: i32, memoryOffset: i32ptr)
• saveReceipt(target: i32ptr, dataPtr: i32ptr, dataLength: i32)
• call(id: u64, dataPtr: i32ptr, dataLength: i32) -> i32 where it returns execution status
• getResponseSize() -> i32 size of the data returned by previous call
• getResponse(offset: i32, length: i32, memoryOffset: i32ptr)
• currentShard() -> u64
• getRecentBeaconStateRoot(slot: i32, memoryOffset: i32ptr)
• getRecentShardStateRoot(slot: i32, memoryOffset: i32ptr)
• abort() to signal failure
• finish() to signal success
• executeCode(codePtr: i32ptr, codeLength: i32, dataPtr: i32ptr, dataLength: i32)

Differences to @vbuterin's version:

• adapted it to the fact that wasm cannot handle 256-bit data natively
• execution input (data) must be retrieved via copyData
• call returns a result code and data must be retrieved separately