Add conversion between ByteVector and FixedBytes

[Akagi201]

It will help to work with alloy crates.

MSRV >= 1.81

The latest alloy-primitives crate requires rustc >= 1.79, and reth requires rustc >= 1.81

https://github.com/alloy-rs/core/blob/main/Cargo.toml#L8 https://github.com/paradigmxyz/reth/blob/main/Cargo.toml#L4