Made clippy happy

[rozbb]

I think a new lint popped up in the last few weeks, so it wasn't caught by the old CI run.

error: importing legacy numeric constants
  --> src/aead.rs:11:51
   |
11 | use core::{default::Default, marker::PhantomData, u8};
   |                                                   ^^
   |
   = help: remove this import
   = note: then `u8::<CONST>` will resolve to the respective associated constant
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#legacy_numeric_constants
   = note: `-D clippy::legacy-numeric-constants` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::legacy_numeric_constants)]`