Memoize helper method instances with Singleton module (#931)

Some helpers, like Rails console's `app`, requires memoization of the
helper's ivars. To support it IRB needs to memoize helper method instances
as well.

lib/irb/helper_method/base.rb

@@ -1,6 +1,10 @@
+require "singleton"
+
 module IRB
   module HelperMethod
     class Base
+      include Singleton
+
       class << self
         def description(description = nil)
           @description = description if description

lib/irb/workspace.rb

@@ -179,7 +179,7 @@ module HelpersContainer
     def self.install_helper_methods
       HelperMethod.helper_methods.each do |name, helper_method_class|
         define_method name do |*args, **opts, &block|
-          helper_method_class.new.execute(*args, **opts, &block)
+          helper_method_class.instance.execute(*args, **opts, &block)
         end unless method_defined?(name)
       end
     end

test/irb/test_helper_method.rb

@@ -97,13 +97,38 @@ def execute
       RUBY
 
       output = run_ruby_file do
-        type <<~INPUT
-          my_helper
-        INPUT
+        type "my_helper"
         type "exit"
       end
 
       assert_include(output, 'Hello from MyHelper')
     end
+
+    def test_helper_method_instances_are_memoized
+      write_ruby <<~RUBY
+        require "irb/helper_method"
+
+        class MyHelper < IRB::HelperMethod::Base
+          description "This is a test helper"
+
+          def execute(val)
+            @val ||= val
+          end
+        end
+
+        IRB::HelperMethod.register(:my_helper, MyHelper)
+
+        binding.irb
+      RUBY
+
+      output = run_ruby_file do
+        type "my_helper(100)"
+        type "my_helper(200)"
+        type "exit"
+      end
+
+      assert_include(output, '=> 100')
+      assert_not_include(output, '=> 200')
+    end
   end
 end