state-res: Refactor lexicographical_topological_sort

Gets rid of unnecessary copying and makes things more explicit by using
a struct with named fields instead of a tuple for tie breaking.

Co-authored-by: Jonathan de Jong <jonathan@automatia.nl>

crates/ruma-state-res/benches/state_res_bench.rs

@@ -48,8 +48,8 @@ fn lexico_topo_sort(c: &mut Criterion) {
             event_id("p") => hashset![event_id("o")],
         };
         b.iter(|| {
-            let _ = state_res::lexicographical_topological_sort(&graph, |id| {
-                Ok((int!(0), MilliSecondsSinceUnixEpoch(uint!(0)), id.to_owned()))
+            let _ = state_res::lexicographical_topological_sort(&graph, |_id| {
+                Ok((int!(0), MilliSecondsSinceUnixEpoch(uint!(0))))
             });
         })
     });