Ractor.allocate should not be allowed

Ractor.allocate and Ractor#dup should not be allowed like Thread.
[Bug #17642]

bootstraptest/test_ractor.rb

@@ -8,6 +8,24 @@ assert_equal 'Ractor', %q{
   Ractor.new{}.class
 }
 
+# Ractor.allocate is not supported
+assert_equal "[:ok, :ok]", %q{
+  rs = []
+  begin
+    Ractor.allocate
+  rescue => e
+    rs << :ok if e.message == 'allocator undefined for Ractor'
+  end
+
+  begin
+    Ractor.new{}.dup
+  rescue
+    rs << :ok if e.message == 'allocator undefined for Ractor'
+  end
+
+  rs
+}
+
 # A Ractor can have a name
 assert_equal 'test-name', %q{
   r = Ractor.new name: 'test-name' do

ractor.c

@@ -2088,6 +2088,8 @@ void
 Init_Ractor(void)
 {
     rb_cRactor = rb_define_class("Ractor", rb_cObject);
+    rb_undef_alloc_func(rb_cRactor);
+
     rb_eRactorError          = rb_define_class_under(rb_cRactor, "Error", rb_eRuntimeError);
     rb_eRactorIsolationError = rb_define_class_under(rb_cRactor, "IsolationError", rb_eRactorError);
     rb_eRactorRemoteError    = rb_define_class_under(rb_cRactor, "RemoteError", rb_eRactorError);