improve merge performance

Signed-off-by: andreas hilti <andreas.hilti@bluewin.ch>

src/CycloneDX.Utils/Merge.cs

@@ -31,12 +31,33 @@ public List<T> Merge(List<T> list1, List<T> list2)
             if (list2 is null) return list1;
 
             var result = new List<T>(list1);
+            // We want to avoid the costly computation of the hashes if possible.
+            // Therefore, we use a nullable type.
+            var resultHashes = new List<int?>(list1.Count);
+            for (int i = 0; i < list1.Count; i++) resultHashes.Add(null);
 
             foreach (var item in list2)
             {
-                if (!(result.Contains(item))) 
+                int hash = item.GetHashCode();
+                bool found = false;
+                for (int i=0; i < result.Count; i++)
+                {
+                    var resultItem = result[i];
+                    if (resultHashes[i] == null)
+                    {
+                        resultHashes[i] = resultItem.GetHashCode();
+                    }
+                    int resultHash = resultHashes[i].Value;
+                    if (hash == resultHash && item.Equals(resultItem))
+                    {
+                        found = true;
+                        break;
+                    }
+                }
+                if (!found) 
                 {
                     result.Add(item);
+                    resultHashes.Add(hash);
                 }
             }