`Enumerator::Chain#rewind` shouldn't rewind elements aren't iterated

### Example:

  ```ruby
  # example.rb
  e = [1]
  def e.rewind; p :r end
  c = e.chain(e)
  c.each{break c}.rewind
  ```

#### Before this patch:

  ```terminal
  $ bin/mruby example.rb
  :r
  :r
  ```

#### After this patch (same as Ruby):

  ```terminal
  $ bin/mruby example.rb
  :r
  ```

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

@@ -17,13 +17,19 @@ class Enumerator
     include Enumerable
 
     def initialize(*args)
-      @enums = args
+      @enums = args.freeze
+      @pos = -1
     end
 
     def each(&block)
-      return to_enum unless block_given?
+      return to_enum unless block
 
-      @enums.each { |e| e.each(&block) }
+      i = 0
+      while i < @enums.size
+        @pos = i
+        @enums[i].each(&block)
+        i += 1
+      end
 
       self
     end
@@ -36,11 +42,10 @@ class Enumerator
     end
 
     def rewind
-      i = @enums.size - 1
-      while 0 <= i
-        e = @enums[i]
+      while 0 <= @pos && @pos < @enums.size
+        e = @enums[@pos]
         e.rewind if e.respond_to?(:rewind)
-        i -= 1
+        @pos -= 1
       end
 
       self

mrbgems/mruby-enum-chain/test/enum_chain.rb

@@ -76,13 +76,24 @@ assert("Enumerator::Chain#size") do
 end
 
 assert("Enumerator::Chain#rewind") do
-  rewound = []
+  rewound = nil
   e1 = [1, 2]
   e2 = (4..6)
-  (class << e1; self end).define_method(:rewind) { rewound << __id__ }
-  (class << e2; self end).define_method(:rewind) { rewound << __id__ }
-  c = e1.chain(e2).each{}.rewind
-  assert_equal [e2.__id__, e1.__id__], rewound
+  (class << e1; self end).define_method(:rewind) { rewound << self }
+  (class << e2; self end).define_method(:rewind) { rewound << self }
+  c = e1.chain(e2)
+
+  rewound = []
+  c.rewind
+  assert_equal [], rewound
+
+  rewound = []
+  c.each{break c}.rewind
+  assert_equal [e1], rewound
+
+  rewound = []
+  c.each{}.rewind
+  assert_equal [e2, e1], rewound
 end
 
 assert("Enumerator::Chain#+") do