Split StableCompare trait out of StableOrd trait.

StableCompare is a companion trait to `StableOrd`. Some types like `Symbol` can be compared in a cross-session stable way, but their `Ord` implementation is not stable. In such cases, a `StableOrd` implementation can be provided to offer a lightweight way for stable sorting. (The more heavyweight option is to sort via `ToStableHashKey`, but then sorting needs to have access to a stable hashing context and `ToStableHashKey` can also be expensive as in the case of `Symbol` where it has to allocate a `String`.)

compiler/rustc_middle/src/ty/typeck_results.rs

@@ -527,7 +527,7 @@ impl<'a, V> LocalTableInContext<'a, V> {
     }
 
     pub fn items_in_stable_order(&self) -> Vec<(ItemLocalId, &'a V)> {
-        self.data.to_sorted_stable_ord()
+        self.data.items().map(|(&k, v)| (k, v)).into_sorted_stable_ord_by_key(|(k, _)| k)
     }
 }