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/value/string.rs

@@ -172,7 +172,7 @@ fn is_valid_nix_identifier(s: &str) -> bool {
             _ => return false,
         }
     }
-    return true;
+    true
 }
 
 /// Escape a Nix string for display, as most user-visible representation