update usage of `U256` from latest `ssz-rs` update

[ralexstokes]

this type was refactored to use `alloy-primitives' impl in ralexstokes/ssz-rs#113

updating this repo should be pretty straightforward with possibly a few API changes (see the ssz-rs PR for some things that changed)

another breaking change was inheriting the included serde impl which encodes a U256 as a hex-string. this deviates from the beacon-apis spec which encodes U256 as a decimal string. we should be able to easily fix this with an #[serde(with = "crate::serde::as_str")] attribute on any type in this repo that uses U256 (e.g. see how we handle Slot)

[Perelyn-sama]

I'll try it

[0xJepsen]

Down to take this over the weekend if you don't get to it @Perelyn-sama