Allow use of DelegateClass in ractor

Use `eval` instead of `define_method` when defining delegate methods for
`DelegateClass`.

Author: luke-gruber

lib/delegate.rb

@@ -344,17 +344,15 @@ def __setobj__(obj)
   end
 end
 
-def Delegator.delegating_block(mid) # :nodoc:
-  prok = lambda do |*args, &block|
+def Delegator.delegating_code(mid) # :nodoc:
+  line = __LINE__+2
+  src = <<~RUBY
+  ruby2_keywords def #{mid}(*args, &block)
     target = self.__getobj__
-    target.__send__(mid, *args, &block)
-  end
-  prok.ruby2_keywords
-  if defined?(Ractor.shareable_proc)
-    Ractor.shareable_proc(&prok)
-  else
-    prok
+    target.__send__(:'#{mid}', *args, &block)
   end
+  RUBY
+  [src, __FILE__, line]
 end
 
 #
@@ -433,7 +431,7 @@ def __setobj__(obj)  # :nodoc:
     class_eval(source.join(";"), __FILE__, __LINE__)
 
     special.each do |method|
-      define_method(method, Delegator.delegating_block(method))
+      module_eval(*Delegator.delegating_code(method))
     end
 
     protected(*protected_instance_methods)