Add `Array#difference` method from Ruby2.6.

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

@@ -124,6 +124,22 @@ class Array
     ary
   end
 
+  ##
+  # call-seq:
+  #    ary.difference(other_ary1, other_ary2, ...)   -> new_ary
+  #
+  # Returns a new array that is a copy of the original array, removing all
+  # occurences of any item that also appear in +other_ary+. The order is
+  # preserved from the original array.
+  #
+  def difference(*args)
+    ary = self.dup
+    args.each do |x|
+      ary = self - x
+    end
+    ary
+  end
+
   ##
   # call-seq:
   #    ary & other_ary      -> new_ary

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

@@ -93,6 +93,14 @@ assert("Array#union") do
   assert_equal [1, 2, 3, 4, 5], a.union(b,c)
 end
 
+assert("Array#difference") do
+  a = [1, 2, 3, 1]
+  b = [1, 4]
+  c = [1, 5]
+
+  assert_equal [2, 3], a.difference(b,c)
+end
+
 assert("Array#&") do
   a = [1, 2, 3, 1]
   b = [1, 4]