Add Queue#each.

I am sure there is an excellent reason `Queue` doesn't support `#each`. But not knowing what that reason is, I went and implemented it. Why? Well, it's awfully convenient for writing simple consumers:

```ruby
inq = Queue.new
outq = Queue.new
doubler = Thread.new do
  inq.each do |n|
    outq << n + n
  end
end
```

It also opens up the possibility of making `Queue` `Enumerable`, but I'm not opening that can of worms in this PR.