* string.c (rb_strseq_index): refactor and avoid
  call of str_strlen() when offset == 0.
  it will improve performance of String#index and #include?

* benchmark/bm_string_index.rb: benchmark for this change

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60086 b2dd03c8-39d4-4d8f-98ff-823fe69b080e