Fix FT events & NEP-297 API improvements

[encody]

Fixes #62

[encody]

Ok now that I'm thinking about it, maybe this could have all been averted by just using a Vec with indirection for the events, like I had in one of the examples. I forgot about that.

#[derive(Serialize)]
pub struct Nep171NftMintData {
    pub owner_id: String,
    pub token_ids: Vec<String>,
}
#[event(standard = "nep171", version = "1.0.0")]
pub enum Nep171 {
    NftMint(Vec<Nep171NftMintData>),
}
let my_event = Nep171::NftMint(vec![Nep171NftMintData {
    owner_id: "owner".to_string(),
    token_ids: vec!["token_1".to_string(), "token_2".to_string()],
}]);

[encody]

A lot of these changes make for a better API, definitely, but maybe the batch isn't necessary.