EVM: system contracts address allocation

[shaunxw]

System contracts requirements: 15 leading zero bytes.

Address allocation.

The rest 5 bytes in the address: 1 category byte ++ 4 address bytes.

Category byte

• 0: Ethereum EVM precompiles.
• 1: Acala precompiles. Set in genesis block.
• 2: Network contracts. Could be deployed later via create_network_contract dispatch call.
• 3: Predeployed ERC20 contracts. Set in genesis block.
• 4: ERC721 contracts. Could be deployed later via create_erc721_contract dispatch call.