Don't clone data by default. User can clone only when relevant.

src/cached.cr

@@ -44,8 +44,12 @@ class DODB::CachedDataBase(V) < DODB::Storage(V)
 		# FIXME: rescues any error the same way.
 		return nil
 	end
+
+	# WARNING: data isn't cloned.
+	# You have to do it yourself in case you modify any value,
+	# otherwise you may encounter problems (at least with indexes).
 	def [](key : Int32) : V
-		@data[key].clone rescue raise MissingEntry.new(key)
+		@data[key] rescue raise MissingEntry.new(key)
 	end
 
 	def []=(index : Int32, value : V)