Extract ActionDispatch::PerformanceTest

I extracted the performance tests and put them into a gem called "rails-perftest" https://github.com/rails/rails-perftest

Work:
* The performance_testing Guide is the README of the new gem
* Extracted performance test code
* Modified the generators
* Extracted `rails benchmarker` and `rails profiler`
* Removed all references to the performance tests

The extracted gem is a drop-in solution. Currently the `perftest` gem's executable makes use of the pwd to determine the `Rails.root`. We can replace this with the new binstubs but I'm not completely sure how to do it :sweat: