This preparation is needed for subsequent
EIPs included in London.
- Add London to Fork enum
- Block number to fork
- Parsing London fork in chain config
- Prepare gas costs table for London
- Prepare EVM opcode dispatcher for London
- Block rewards for London
- Prepare hive script for London
instead of using stdlib/json, now we switch to json_serialization
the result is much tidier code and more robust when parsing
optional fields.
fixes#635
jangko