Fix serialization issues for Forest RPC untagged enums.

[elmattic]

Reproduce by using #57 and uncomment one of the EthGetBalance tests:

cargo run -q --bin cna-test-suite run src/test-suite/config.json.ignoreme
0	ChainHead with no authorization token	passed
1	EthAccounts with no authorization token	passed
2	EthAddressToFilecoinAddress with no authorization token	passed
3	EthBlockNumber with no authorization token	passed
4	EthChainId with no authorization token	passed
5	EthGasPrice with no authorization token	passed
6	EthGetBalance with no authorization token and subtype_2	failed
	0	err
	couldn't serialize params
	can only flatten structs and maps (got a string)
	1	src/test-suite/src/tests.rs:158:9
7	EthGetBalance with no authorization token and subtype_3	passed
8	EthGetBalance with no authorization token and subtype_4	passed
9	EthNewBlockFilter with no authorization token	passed
10	EthNewPendingTransactionFilter with no authorization token	passed
11	EthProtocolVersion with no authorization token	passed
12	EthSyncing with no authorization token	passed
failure 0 (EthGetBalance with no authorization token and subtype_2)
	0	err
	couldn't serialize params
	can only flatten structs and maps (got a string)
	1	src/test-suite/src/tests.rs:158:9
0 skipped, 12 succeeded, 1 failed

[elmattic]

Maybe one approach would be to guide code generation to use an enum:

So instead of having:

#[derive(Clone, Debug, Deserialize, Serialize)]
pub struct BlockNumberOrHash {
    #[serde(flatten, default, skip_serializing_if = "Option::is_none")]
    pub subtype_0: Option<String>,
    ...
}

We would like:

#[derive(Clone, Debug, Deserialize, Serialize)]
#[serde(untagged)]
pub enum BlockNumberOrHash {
    Subtype0(String),
    ...
}

[elmattic]

update: I've created an issue on the typify repository to ask if there's an easy workaround.