Optimize `chars2bytes` with `MRB_UTF8_STRING` to ASCII only string

### Benchmark (with `MRB_UTF8_STRING`)

  ```
  $ mruby -e '
    COUNT = 150000
    SIZE = 10000
    strs = Array.new(COUNT) do
      s = "a" * SIZE
      s.size  # set `MRB_STR_ASCII` flag
      s
    end
    i = 0
    t = Time.now
    while i < COUNT
      strs[i][-2..-1] = ""
      i += 1
    end
    printf "%.2f sec\n", Time.now - t
  '

  1.10 sec  # before
  0.07 sec  # after
  ```

0 files changed, 0 insertions, 0 deletions