update rust setup

[wminshew]

ethereum_types::U256 is basically the same as primitive_types::U256 but later on it will allow you to #[derive(Deserialize)] directly from the json. I spent a lot of time learning how serde worked & trying to build around the fact that primitive_types::U256 couldn't do this and.. well, eventually gave up. Rust doesn't make that kind of work around trivial (afaict..)

my original solution used Option<String> everywhere and it obviously makes the code significantly uglier / harder

lmk if you want to chat more about any of the other changes but think the rest are more straightforward