Add `AlgebraicType/Value::U256`

[Centril]

Many cryptographic hashes result in 32 byte (256 bit) outputs, and storing this as a Box<U256> is about as cheap as we can make it. This new variant/type would be helpful for e.g., Identity.

[Centril]

Should probably use https://docs.rs/ethnum/latest/ethnum/index.html.