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

### Benchmark (with `MRB_UTF8_STRING`)

  ```ruby
  # benchmark.rb
  COUNT = 300000
  SIZE = 10000
  s = "a" * SIZE
  s.size  # set `MRB_STR_ASCII` flag
  i = 0
  while i < COUNT
    s[-1]
    i += 1
  end
  ```

#### Before this patch:

  ```
  $ time mruby benchmark.rb
          2.06 real         2.05 user         0.00 sys
  ```

#### After this patch:

  ```
  $ time mruby benchmark.rb
          0.05 real         0.04 user         0.00 sys
  ```

0 files changed, 0 insertions, 0 deletions