Improve performance of #uniq across a large number of nodes

Been chasing performance regressions in arel after upgrading to rails 4. This fixes 

https://github.com/rails/arel/issues/205

Before:

![image](https://f.cloud.github.com/assets/5213/1055453/31361996-1139-11e3-9b09-fe3cda6561aa.png)


After:

![image](https://f.cloud.github.com/assets/5213/1055468/a0452cb4-1139-11e3-8c01-4f9980e87161.png)


This is a mild improvement, I am chasing 10 missing milliseconds so every one counts :) 

For context, Rails 3 has a way faster arel ... here is the same page in Rails 3. 

![image](https://f.cloud.github.com/assets/5213/1055478/193dc81a-113a-11e3-9ef7-6d915db6f5ef.png)