Unification bug with recursive types and unions

``` ruby
generic class Entry
  def next=(n)
    @next = n
  end

  def next
    @next
  end
end

entry1 = Entry.new
entry1.next = entry1

entry2 = Entry.new
entry2.next = entry1
entry2.next = entry2

union = entry1
union = entry2

union.next
```

The type inference gives two Entry classes, but only one should exist, that has @next pointing to itself
