set_over_array for rollout gem

rollout_perf_spec.rb

require 'benchmark'

describe "Rollout::Feature" do
  describe "performance" do
    before do
      @feature = Rollout::Feature.new(:chat)
    end

    it "outputs result" do
      puts Benchmark.measure {
        # 1000 enabled users
        (1..1000).each do |num|
          @feature.add_user num
        end

        # 1000000 lookups
        (1..1000000).each do |num|
          @feature.active?(:chat, num)
        end
      }
    end
  end
end

simple_benchmark_result.md

users (with 1M lookup)	With Array (sec)	With Set (sec)
100	5.411557	3.558224
1000	39.546319	3.583432