Merge pull request #4256 from shuujii/move-none-to-mrblib-enum

Move `NONE` to `mrblib/enum.rb`

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

@@ -266,7 +266,6 @@ class Array
     self
   end
 
-  NONE=Object.new
   ##
   #  call-seq:
   #     ary.fetch(index)                    -> obj

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

@@ -205,7 +205,6 @@ module Enumerable
     ary.collect{|e,i| orig[i]}
   end
 
-  NONE = Object.new
   ##
   # call-seq:
   #    enum.first       ->  obj or nil

mrblib/enum.rb

@@ -13,6 +13,8 @@
 # @ISO 15.3.2
 module Enumerable
 
+  NONE = Object.new
+
   ##
   # Call the given block for each element
   # which is yield by +each+. Return false