Cleanup web3/rpc/eth type converters

[jangko]

Currently web3/rpc/eth type converters are scattered around in many places.
Not all the conversion are written using converters. And they appear ugly.
Duplicates are common. It's time to clean them up and gather them together.
Ugly codes need to be rewritten using proper converters.
New intermediate types may need to be invented, such as RPCReceipt as a complement to regular Receipt/ReceiptObject type.
RPC types also need to be rewritten using saner web3 primitives.

PS: the converter we are talking about here is not Nim converter, but a func or proc.

• rpc/merge utils
• engine_api
• pyspec_sim
• engine_sim
• sealer
• json_rpc_data_source
• rpc_types
• rpc_utils
• execution_types