Resolve legacy_numeric_constants clippy lints

    warning: importing legacy numeric constants
     --> src/layout.rs:1:17
      |
    1 | use core::{mem, usize};
      |                 ^^^^^
      |
      = help: remove this import
      = note: then `usize::<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: `-W clippy::legacy-numeric-constants` implied by `-W clippy::all`
      = help: to override `-W clippy::all` add `#[allow(clippy::legacy_numeric_constants)]`

src/layout.rs

@@ -1,4 +1,4 @@
-use core::{mem, usize};
+use core::mem;
 
 #[doc(hidden)]
 pub struct Layout<T: ?Sized>(T);