perf: remove more redundant scanning (pubgrub-rs#251)

Doing the stronger version of pubgrub-rs#175 because it turns out to be important to cargo crates.

src/internal/core.rs

@@ -128,10 +128,9 @@ impl<DP: DependencyProvider> State<DP> {
                     Relation::AlmostSatisfied(package_almost) => {
                         // Add `package_almost` to the `unit_propagation_buffer` set.
                         // Putting items in `unit_propagation_buffer` more than once waste cycles,
-                        // but so does checking for duplicates.
-                        // In practice the most common pathology is adding the same package repeatedly.
-                        // So we only check if it is duplicated with the last item.
-                        if self.unit_propagation_buffer.last() != Some(&package_almost) {
+                        // but so does allocating a hash map and hashing each item.
+                        // In practice `unit_propagation_buffer` is small enough that we can just do a linear scan.
+                        if !self.unit_propagation_buffer.contains(&package_almost) {
                             self.unit_propagation_buffer.push(package_almost.clone());
                         }
                         // Add (not term) to the partial solution with incompat as cause.