Improved performance of String#each_char.

Thanks to @dbussink for hinting about this solution.

rubinius-benchmark - core/string/bench_each_char.rb

Before:
  String#each_char mixed UTF-8 string
                          36420.2 (±2.2%) i/s -     185076 in   5.084328s

After:
  String#each_char mixed UTF-8 string
                          95467.5 (±2.3%) i/s -     482496 in   5.056702s