* benchmark/bm_hash_flatten.rb: added. r43896 is about 4 times faster
  than 2.0.0p353.

* benchmark/bm_hash_keys.rb: added. r43896 is about 5 times faster
  than 2.0.0p353.

* benchmark/bm_hash_values.rb: added. r43896 is about 5 times faster
  than 2.0.0p353.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43898 b2dd03c8-39d4-4d8f-98ff-823fe69b080e