Function assert_nat_lit

crates/kernel/src/whnf.rs:4765–4774

(e: &AE, expected: u64)

    let addr = Address::hash(&v.to_le_bytes());
    super::super::testing::ME::nat(v, addr)
  }

  /// Build a Nat env with Nat, Nat.zero, Nat.succ, Nat.rec, and Nat.sub.
  /// Nat.sub is defined as a primitive that the kernel's try_reduce_nat handles,
  /// but also has a delta-unfoldable body using Nat.rec (to test reduction order).
  fn nat_env() -> KEnv<Anon> {
    use super::super::constant::RecRule;

    let mut env = KEnv::new();
    let block = mk_id("Nat");

    // Nat : Sort 1
    env.insert(