Sized enumerator

[marcandre]

This PR adds a lazy size block to enumerators returned by Batches#find_each, find_in_batches, Result#each and Enumerable#index_by.

This allows, for example using my gem with_progress:

Users.where("balance > 0").find_each.with_progress do |u|
  UserMailer.send_invoice(u)
end

This will print a progress bar with an estimated time left, etc, while still loading the records in batches.

The PR also has a couple of test cleanup commits at the beginning.

Note: In Ruby 1.9, there is no Enumerator#size, so the test is conditional on that. The block given to to_enum is ignored in 1.9, so no specific treatment is needed.

[rafaelfranca]

@marcandre seems good to me. Could you squash the commits that add the feature? The two obsolete removals commits are good as separated commits.

We will need a CHANGELOG entry too.

[marcandre]

Right, forgot about the CHANGELOG, sorry. Done.
I feel the commits are independent and better left unsquashed. More detailed explanation here.

[rafaelfranca]

Thanks.

About the squash it is fine to me. I agree with you that they are independent.