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/attrs.rs

@@ -321,7 +321,6 @@ impl NixAttrs {
             }
         }
 
-        // TODO(tazjin): extend_reserve(count) (rust#72631)
         let mut attrs = NixAttrs(AttrsRep::Map(BTreeMap::new()));
 
         for _ in 0..count {