Improve the performance of MultipleMemoizedHelpers

Fetching memoized helpers for each of the ancestors is not optimal, is
not optimal and has non-linear complexity.

lib/rubocop/cop/rspec/multiple_memoized_helpers.rb

@@ -100,8 +100,14 @@ def on_block(node)
           add_offense(node, message: format(MSG, count: count, max: max))
         end
 
+        def on_new_investigation
+          @example_group_memoized_helpers = {}
+        end
+
         private
 
+        attr_reader :example_group_memoized_helpers
+
         def all_helpers(node)
           [
             *helpers(node),
@@ -110,13 +116,14 @@ def all_helpers(node)
         end
 
         def helpers(node)
-          variable_nodes(node).map do |variable_node|
-            if variable_node.block_type?
-              variable_definition?(variable_node.send_node)
-            else # block-pass (`let(:foo, &bar)`)
-              variable_definition?(variable_node)
+          @example_group_memoized_helpers[node] ||=
+            variable_nodes(node).map do |variable_node|
+              if variable_node.block_type?
+                variable_definition?(variable_node.send_node)
+              else # block-pass (`let(:foo, &bar)`)
+                variable_definition?(variable_node)
+              end
             end
-          end
         end
 
         def variable_nodes(node)