Use 32/64-bit digits, depending on the system

[OTheDev]

No description provided.

[OTheDev]

Some tests rely on the existence of primitive integer types with 4-times the width of a digit. For u32 digits, we have i128 and u128. For u64 digits, we'll try using alloy_primitives::{U256, I256}. These will only be a dev dependency.