feat(tvix/eval): Implement builtins.mapAttrs

I played around a little bit with doing this in-place, but ended up
going with this perhaps slightly clone-heavy approach for now because
ideally most clones on Value are cheap - but later we should benchmark
alternate approaches that get to reuse allocations better if necessary
or possible.

Change-Id: If998eb2056cedefdf2fb480b0568ac8329ccfc44
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7068
Autosubmit: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/mod.rs

@@ -435,6 +435,19 @@ fn pure_builtins() -> Vec<Builtin> {
                 .map(|list| Value::List(NixList::from(list)))
                 .map_err(Into::into)
         }),
+        Builtin::new(
+            "mapAttrs",
+            &[true, true],
+            |args: Vec<Value>, vm: &mut VM| {
+                let attrs = args[1].to_attrs()?;
+                let mut res = BTreeMap::new();
+                for (key, value) in attrs.as_ref() {
+                    let value = vm.call_with(&args[0], [key.clone().into(), value.clone()])?;
+                    res.insert(key.clone(), value);
+                }
+                Ok(Value::attrs(NixAttrs::from_map(res)))
+            },
+        ),
         Builtin::new(
             "match",
             &[true, true],