Should clarify the role of `mruby-kernel-ext` and `mruby-object-ext`; close #4449

The former should contain function like methods, and the latter should
contain methods shared by all objects.

mrbgems/mruby-kernel-ext/mrblib/kernel.rb

-module Kernel
-  # call-seq:
-  #   obj.yield_self {|_obj|...} -> an_object
-  #   obj.then {|_obj|...}       -> an_object
-  #
-  # Yields <i>obj</i> and returns the result.
-  #
-  #   'my string'.yield_self {|s|s.upcase} #=> "MY STRING"
-  #
-  def yield_self(&block)
-    return to_enum :yield_self unless block
-    block.call(self)
-  end
-  alias then yield_self
-end

mrbgems/mruby-kernel-ext/src/kernel.c

@@ -206,22 +206,6 @@ mrb_f_hash(mrb_state *mrb, mrb_value self)
   return mrb_ensure_hash_type(mrb, arg);
 }
 
-/*
- *  call-seq:
- *     obj.itself -> an_object
- *
- *  Returns <i>obj</i>.
- *
- *      string = 'my string' #=> "my string"
- *      string.itself.object_id == string.object_id #=> true
- *
- */
-static mrb_value
-mrb_f_itself(mrb_state *mrb, mrb_value self)
-{
-  return self;
-}
-
 void
 mrb_mruby_kernel_ext_gem_init(mrb_state *mrb)
 {
@@ -237,7 +221,6 @@ mrb_mruby_kernel_ext_gem_init(mrb_state *mrb)
   mrb_define_module_function(mrb, krn, "String", mrb_f_string, MRB_ARGS_REQ(1));
   mrb_define_module_function(mrb, krn, "Array", mrb_f_array, MRB_ARGS_REQ(1));
   mrb_define_module_function(mrb, krn, "Hash", mrb_f_hash, MRB_ARGS_REQ(1));
-  mrb_define_module_function(mrb, krn, "itself", mrb_f_itself, MRB_ARGS_NONE());
 }
 
 void

mrbgems/mruby-object-ext/mrblib/object.rb

-class Object
+module Kernel
+  # call-seq:
+  #   obj.yield_self {|_obj|...} -> an_object
+  #   obj.then {|_obj|...}       -> an_object
+  #
+  # Yields <i>obj</i> and returns the result.
+  #
+  #   'my string'.yield_self {|s|s.upcase} #=> "MY STRING"
+  #
+  def yield_self(&block)
+    return to_enum :yield_self unless block
+    block.call(self)
+  end
+  alias then yield_self
+
   ##
   #  call-seq:
   #     obj.tap{|x|...}    -> obj

mrbgems/mruby-object-ext/src/object.c

@@ -44,6 +44,22 @@ nil_to_i(mrb_state *mrb, mrb_value obj)
   return mrb_fixnum_value(0);
 }
 
+/*
+ *  call-seq:
+ *     obj.itself -> an_object
+ *
+ *  Returns <i>obj</i>.
+ *
+ *      string = 'my string' #=> "my string"
+ *      string.itself.object_id == string.object_id #=> true
+ *
+ */
+static mrb_value
+mrb_f_itself(mrb_state *mrb, mrb_value self)
+{
+  return self;
+}
+
 /*
  *  call-seq:
  *     obj.instance_exec(arg...) {|var...| block }                       -> obj
@@ -102,6 +118,7 @@ mrb_mruby_object_ext_gem_init(mrb_state* mrb)
   mrb_define_method(mrb, n, "to_f", nil_to_f,       MRB_ARGS_NONE());
 #endif
   mrb_define_method(mrb, n, "to_i", nil_to_i,       MRB_ARGS_NONE());
+  mrb_define_module_function(mrb, mrb->kernel_module, "itself", mrb_f_itself, MRB_ARGS_NONE());
 
   mrb_define_method(mrb, mrb->kernel_module, "instance_exec", mrb_obj_instance_exec, MRB_ARGS_ANY() | MRB_ARGS_BLOCK());
 }