Implement Enumerator#+ and Enumerable#chain [Feature #15144]

They return an Enumerator::Chain object which is a subclass of
Enumerator, which represents a chain of enumerables that works as a
single enumerator.

```ruby
e = (1..3).chain([4, 5])
e.to_a #=> [1, 2, 3, 4, 5]

e = (1..3).each + [4, 5]
e.to_a #=> [1, 2, 3, 4, 5]
```

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65949 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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