Replace `B256` aliases with `u8` arrays `B256([u8;32])`

Hi @recmo are you open to replacing current `B` types that are aliases for `U` with [u8;32], we will do the legwork if you approve it.

Current B types in revm that i need are made with `primitive-types` macros and have some From/Serde added to them:
https://github.com/bluealloy/revm/blob/afc30663270f77df9b4399ad9d4cfb0ad2b814ec/crates/primitives/src/bits.rs#L4-L21

This would make a lot of projects be able to directly import ruint: ethers, reth, foundry, revm.