fix(tvix/eval): fix current clippy warnings

It's been a while since the last time, so quite a lot of stuff has
accumulated here.

Change-Id: I0762827c197b30a917ff470fd8ae8f220f6ba247
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7597
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/lib.rs

@@ -223,7 +223,7 @@ impl<'code, 'co, 'ro> Evaluation<'code, 'co, 'ro> {
                     None
                 }
             })
-            .unwrap_or_else(|| Default::default());
+            .unwrap_or_default();
 
         let runtime_observer = self.runtime_observer.take().unwrap_or(&mut noop_observer);
         let vm_result = run_lambda(