eth,rpc: make RPCBlock,RPCTransaction,RPCReceipt as public struct

[jsvisa]

This PR moves the RPC Response-related structures, which were originally in the internal/ethapi package, to the rpc package.
This allows third-party clients to access more raw data based on these structures. For example, ethclient.BlockByNumber can only return the types.Block structure:

go-ethereum/ethclient/ethclient.go

Lines 84 to 86 in 9e8575c

	func (ec *Client) BlockByHash(ctx context.Context, hash common.Hash) (*types.Block, error) {
	return ec.getBlock(ctx, "eth_getBlockByHash", hash, true)
	}

But on RPC server's side, we do return more data (such as totalDifficulty) in rpcMarshalBlock.

go-ethereum/internal/ethapi/api.go

Lines 1234 to 1243 in 9e8575c

	func (s *BlockChainAPI) rpcMarshalBlock(ctx context.Context, b *types.Block, inclTx bool, fullTx bool) (*rpc.RPCBlock, error) {
	block, err := rpc.RPCMarshalBlock(b, inclTx, fullTx, s.b.ChainConfig())
	if err != nil {
	return nil, err
	}
	if inclTx {
	block.TotalDifficulty = (*hexutil.Big)(s.b.GetTd(ctx, b.Hash()))
	}
	return block, err
	}

[fjl]

I'm against adding these in package rpc, and here's why:

• The ethclient API does not strictly follow the RPC API, and instead provides the data structures as used by go-ethereum core. The idea with this is that code using go-ethereum should only deal with one set of types, namely the ones defined in core/types. This should ensure most code can be used regardless of whether it accesses the blockchain using RPC, or using a p2p node in the same process.

• package rpc is not a good place for these types. The rpc package is supposed to be a JSON-RPC client/server. Ideally, there should be no Ethereum references in this package. We do have BlockNumber in there, but it's for legacy reasons and BlockNumber isn't a complicated type.

[jsvisa]

@fjl Thank you for the detailed explanation. And I have some other questions:

1. If ethclient is not the standard RPC client, are there any RPC clients in Golang, an alternative to web3.py, web3.js?
2. What do you think of introducing several structures such as RPCBlock in the ethapi module to replace map[string]interface{}?
3. And what do you think of puting ethapi module outside of internal is a good idea(or why wasn't it out there before)?