Add Enumerator::Product from marcandre/backports

See marcandre/backports#192 for a few
small additional fixes incorporated here.

core/src/main/ruby/jruby/kernel/enumerator.rb

@@ -467,4 +467,55 @@ def __memo_set(yielder, value)
       yielder.instance_variable_set :@__memo, value
     end
   end
-end
+end
+
+# From https://github.com/marcandre/backports, MIT license
+def Enumerator.product(*enums, **kwargs, &block)
+  if kwargs && !kwargs.empty?
+    raise ArgumentError.new("unknown keywords: " + kwargs.keys.map(&:inspect).join(", "))
+  end
+  Enumerator::Product.new(*enums).each(&block)
+end
+
+class Enumerator::Product < Enumerator
+  def initialize(*enums, **nil)
+    @__enums = enums
+  end
+
+  def each(&block)
+    return self unless block
+    __execute(block, [], @__enums)
+  end
+
+  def __execute(block, values, enums)
+    if enums.empty?
+      block.yield values
+    else
+      enum, *enums = enums
+      enum.each_entry do |val|
+        __execute(block, [*values, val], enums)
+      end
+    end
+    nil
+  end
+  private :__execute
+
+  def size
+    total_size = 1
+    @__enums.each do |enum|
+      return nil if !enum.respond_to?(:size)
+      size = enum.size
+      return size if size == nil || size == Float::INFINITY || size == -Float::INFINITY
+      return nil if !size.kind_of?(Integer) || size.equal?(Float::NAN)
+      total_size *= size
+    end
+    total_size
+  end
+
+  def rewind
+    @__enums.each do |enum|
+      enum.rewind if enum.respond_to?(:rewind)
+    end
+    self
+  end
+end