use const generics in DICE cert trait

This interface would ideally return sized arrays for the generic 'get_range' function. This isn't yet possible w/o using unstable features:

trait Foo {
    const BAR: usize;
    
    fn baz(self) -> [u8; Self::BAR];
}

Currently this produces an error:

error: generic parameters may not be used in const operations
 --> src/lib.rs:6:26
  |
6 |     fn baz(self) -> [u8; Self::BAR];
  |                          ^^^^^^^^^ cannot perform const operation using `Self`
  |
  = note: type parameters may not be used in const expressions
  = help: use `#![feature(generic_const_exprs)]` to allow generic const expressions