message: allow clippy::suspicious_arithmetic_impl for BigInt
diff: +// `Add`/`Sub` ops may flip from `BigInt` to its `BigUint` magnitude
  +#![allow(clippy::suspicious_arithmetic_impl)]
  +
  #[cfg(feature = "quickcheck")]
  use crate::std_alloc::Box;
  use crate::std_alloc::{String, Vec};
  