[Performance] Don't create fibers while merging bind variables

Master branch has a serious performance regression when including objects for a relation. e.g. ```Item.includes(:people)```.

The regression is caused by the use of an enumerator in the relation merger.rb, see https://github.com/rails/rails/blob/master/activerecord/lib/active_record/relation/merger.rb#L110

This causes ruby to create many fibers (6309 according to ruby-prof call graph) while iterating, when the same effect could be achieved by changing how the bind_values enumerable is accessed. See pull request.

Here are my test results for my own project
```
Rails 3.2.13
Item.include(:people).limit(1000) #=> 1.049828

Rails 4 4-0-stable
Item.include(:people).limit(1000) #=> 3.609199

Rails 4 master
Item.include(:people).limit(1000) #=> 31.08081

Rails 4 master with optimization (pull request)
Item.include(:people).limit(1000) #=> 2.134169
```
Obviously there is still a performance regression between 3.2.13 and Rails 4.x, but at least this fix brings it back to a reasonable level. I'll look into the remaining performance hit soon.

Use the following gist to test performance on various versions of rails.
https://gist.github.com/njakobsen/6393783