Add enumerator chain feature (CRuby-2.6 compatible)

- Enumerator::Chain
  - Enumerable#chain
  - Enumerable#+

mrbgems/mruby-enum-chain/mrbgem.rake

@@ -0,0 +1,6 @@
+MRuby::Gem::Specification.new('mruby-enum-chain') do |spec|
+  spec.license = 'MIT'
+  spec.author  = 'mruby developers'
+  spec.summary = 'Enumerator::Chain class'
+  spec.add_dependency('mruby-enumerator', :core => 'mruby-enumerator')
+end

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

@@ -0,0 +1,60 @@
+##
+# chain.rb Enumerator::Chain class
+# See Copyright Notice in mruby.h
+
+module Enumerable
+  def chain(*args)
+    Enumerator::Chain.new(self, *args)
+  end
+
+  def +(other)
+    Enumerator::Chain.new(self, other)
+  end
+end
+
+class Enumerator
+  class Chain
+    include Enumerable
+
+    def initialize(*args)
+      @enums = args
+    end
+
+    def initialize_copy(orig)
+      @enums = orig.__copy_enums
+    end
+
+    def each(&block)
+      return to_enum unless block_given?
+
+      @enums.each { |e| e.each(&block) }
+
+      self
+    end
+
+    def size
+      @enums.reduce(0) do |a, e|
+        return nil unless e.respond_to?(:size)
+        a + e.size
+      end
+    end
+
+    def rewind
+      @enums.reverse_each do |e|
+        e.rewind if e.respond_to?(:rewind)
+      end
+
+      self
+    end
+
+    def inspect
+      "#<#{self.class}: #{@enums.inspect}>"
+    end
+
+    def __copy_enums
+      @enums.each_with_object([]) do |e, a|
+        a << e.clone
+      end
+    end
+  end
+end