[TreeDictionary] merge: Add missing invariant checks

The lack of these calls allowed our tests to pass even though the
implementation is failing to properly update the count. D’oh!

This and the previous commits adds a myriad new test failures.

Sources/HashTreeCollections/TreeDictionary/TreeDictionary+Merge.swift

@@ -43,7 +43,8 @@ extension TreeDictionary {
     uniquingKeysWith combine: (Value, Value) throws -> Value
   ) rethrows {
     _invalidateIndices()
-    try _root.merge(.top, keysAndValues._root, combine)
+    _ = try _root.merge(.top, keysAndValues._root, combine)
+    _invariantCheck()
   }
 
   /// Merges the key-value pairs in the given sequence into the dictionary,
@@ -87,6 +88,7 @@ extension TreeDictionary {
         }
       }
     }
+    _invariantCheck()
   }
 
   /// Merges the key-value pairs in the given sequence into the dictionary,