Fix `Module#dup` to frozen module

Before this patch:

  $ bin/mruby -e 'p Module.new.freeze.dup.frozen?'  #=> true

After this patch (same as Ruby):

  $ bin/mruby -e 'p Module.new.freeze.dup.frozen?'  #=> false

src/class.c

@@ -2077,6 +2077,14 @@ mrb_mod_eqq(mrb_state *mrb, mrb_value mod)
   return mrb_bool_value(eqq);
 }
 
+static mrb_value
+mrb_mod_dup(mrb_state *mrb, mrb_value self)
+{
+  mrb_value mod = mrb_obj_clone(mrb, self);
+  MRB_UNSET_FROZEN_FLAG(mrb_obj_ptr(mod));
+  return mod;
+}
+
 static mrb_value
 mrb_mod_module_function(mrb_state *mrb, mrb_value mod)
 {
@@ -2207,6 +2215,7 @@ mrb_init_class(mrb_state *mrb)
   mrb_define_method(mrb, mod, "method_defined?",         mrb_mod_method_defined,   MRB_ARGS_REQ(1)); /* 15.2.2.4.34 */
   mrb_define_method(mrb, mod, "define_method",           mod_define_method,        MRB_ARGS_ARG(1,1));
   mrb_define_method(mrb, mod, "===",                     mrb_mod_eqq,              MRB_ARGS_REQ(1)); /* 15.2.2.4.7 */
+  mrb_define_method(mrb, mod, "dup",                     mrb_mod_dup,              MRB_ARGS_NONE());
 
   mrb_undef_method(mrb, cls, "append_features");
   mrb_undef_method(mrb, cls, "extend_object");

src/kernel.c

@@ -805,5 +805,4 @@ mrb_init_kernel(mrb_state *mrb)
   mrb_define_method(mrb, krn, "__to_str",                   mrb_to_str,                      MRB_ARGS_NONE()); /* internal */
 
   mrb_include_module(mrb, mrb->object_class, mrb->kernel_module);
-  mrb_define_alias(mrb, mrb->module_class, "dup", "clone"); /* XXX */
 }

test/t/module.rb

@@ -400,6 +400,29 @@ end
 
 # Not ISO specified
 
+assert('Module#dup') do
+  module TestModuleDup
+    @@cvar = :cvar
+    class << self
+      attr_accessor :cattr
+      def cmeth; :cmeth end
+    end
+    def cvar; @@cvar end
+    def imeth; :imeth end
+    self.cattr = :cattr
+  end
+
+  m = TestModuleDup.dup
+  o = Object.include(m).new
+  assert_equal(:cattr, m.cattr)
+  assert_equal(:cmeth, m.cmeth)
+  assert_equal(:cvar, o.cvar)
+  assert_equal(:imeth, o.imeth)
+  assert_match("#<Module:0x*>", m.to_s)
+  assert_not_predicate(m, :frozen?)
+  assert_not_predicate(TestModuleDup.freeze.dup, :frozen?)
+end
+
 assert('Module#define_method') do
   c = Class.new {
     define_method(:m1) { :ok }