Add a simple JSON.dump and JSON.load benchmark

* Restoring benchmarks/ohai.json from dad9fbf.

benchmarks/bench.rb

@@ -0,0 +1,41 @@
+require 'benchmark/ips'
+
+$:.unshift File.expand_path('../ext', __dir__)
+$:.unshift File.expand_path('../lib', __dir__)
+
+bench, mode = ARGV
+
+if mode == 'pure'
+  require 'json/pure'
+else
+  require 'json/ext'
+end
+
+bench_dump = bench == 'dump'
+if bench_dump
+  p JSON.generator
+else
+  p JSON.parser
+end
+
+str = File.read("#{__dir__}/ohai.json")
+obj = JSON.load(str)
+
+Benchmark.ips do |x|
+  unless RUBY_ENGINE == 'ruby'
+    x.warmup = 5
+    x.iterations = 5
+  end
+
+  if bench_dump
+    x.report('JSON.dump(obj)') do # max_nesting: false, allow_nan: true
+      JSON.dump(obj)
+    end
+  else
+    x.report('JSON.load(str)') do # max_nesting: false, allow_nan: true, allow_blank: true, create_additions: true
+      JSON.load(str)
+    end
+  end
+
+  x.compare!
+end