MONGOID-5391 - Add #pluck_each, extract out PluckEnumerator

[johnnyshields]

Fixes MONGOID-5391

I'd like to humbly request this feature for reconsideration to be merged. A certain Mongoid maintainer of xmas past rejected this on the grounds that there are plans in MONGOID-5411 to make a feature which returns query raw results as a demongoized, field-name-aliased Hash. I fully support doing MONGOID-5411, however its timeline is unspecified (still in "Needs Triage" state), whereas this feature (#pluck_each) is ready to be used today. It yields a tremendous performance gain 🚀 and I've gotten a ton of mileage out of it in my app.

I also feel that this feature is not mutually exclusive functionality-wise with MONGOID-5411. Although the API syntax of MONGOID-5411 is not yet defined, if you just want to pluck a single value, it will look something like .only(:my_field).query_as_hash.each {|result| result[:my_field] } whereas the same in this code can be done here far more tersely as .pluck_each(:my_field) {|value| ... }

Lastly, I think the extraction of PluckEnumerator in this PR is probably a stepping stone toward implementing MONGOID-5411. I'd be happy to add looking at MONGOID-5411 to my queue.

For reference, the previously closed PR including discussion is here: #5332

[johnnyshields]

@alexbevi @Neilshweky I really feel this PR should be merged. I find myself using it constantly in my app, it leads to a signficant Ruby performance gain.

[alexbevi]

Hi @johnnyshields, this PR (and associated ticket) remains on our shortlist for continued discussion. I hope to have further information for you soon.

[johnnyshields]

Merged into Mongoid Ultra 🌈