Added benchmark to compare with ObjectSpace

bench/benchmark_helper.rb

+$:.unshift(File.expand_path('../../lib', __FILE__))
+
+require 'allocations'
+require 'benchmark/ips'

bench/compared_with_object_space_bench.rb

+require_relative 'benchmark_helper'
+
+Allocations.start
+
+strings = 10_000.times.map { 'a' * 32 }
+
+Benchmark.ips do |bench|
+  bench.report 'ObjectSpace' do
+    ObjectSpace.each_object(String).count
+  end
+
+  bench.report 'Allocations' do
+    Allocations.to_hash[String]
+  end
+
+  bench.compare!
+end
+
+Allocations.stop