* string.c (rb_str_split_m): improve splitting into chars by an
  empty string, without a regexp.

    Comparison:
                           to_chars-1
              built-ruby:   1273527.6 i/s
            compare-ruby:    189423.3 i/s - 6.72x  slower

                          to_chars-10
              built-ruby:    120993.5 i/s
            compare-ruby:     37075.8 i/s - 3.26x  slower

                         to_chars-100
              built-ruby:     15646.4 i/s
            compare-ruby:      4012.1 i/s - 3.90x  slower

                        to_chars-1000
              built-ruby:      1295.1 i/s
            compare-ruby:       408.5 i/s - 3.17x  slower

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@67582 b2dd03c8-39d4-4d8f-98ff-823fe69b080e