There are numerous ruby tools that need to recursively scan
the project directory, such as Zeitwerk, rubocop, etc.

All of them end up listing childs of a directory then for each child
emit a `stat` call to check if it's a directory or not.

This is common enough for a pattern that on most operating
systems, `struct dirent` include a `dtype` member that allows to
check the file type without issuing a any extra system calls.

By yielding that type, we can make these routines twice as fast.

```
$ hyperfine './miniruby --disable-all --yjit ../test.rb' 'OPT=1 ./miniruby --disable-all --yjit ../test.rb'
Benchmark 1: ./miniruby --disable-all --yjit ../test.rb
  Time (mean ± σ):      1.428 s ±  0.062 s    [User: 0.342 s, System: 1.070 s]
  Range (min … max):    1.396 s …  1.601 s    10 runs

Benchmark 2: OPT=1 ./miniruby --disable-all --yjit ../test.rb
  Time (mean ± σ):     673.8 ms ±   5.8 ms    [User: 146.0 ms, System: 527.3 ms]
  Range (min … max):   659.7 ms … 679.6 ms    10 runs

Summary
  OPT=1 ./miniruby --disable-all --yjit ../test.rb ran
    2.12 ± 0.09 times faster than ./miniruby --disable-all --yjit ../test.rb
```

```ruby
if ENV['OPT']
  def count_ruby_files
    count = 0
    queue = [File.expand_path(__dir__)]
    while dir = queue.pop
      Dir.each_child(dir) do |name, type|
        next if name.start_with?(".")

        case type
        when :directory
          queue << File.join(dir, name)
        when :file
          count += 1 if name.end_with?(".rb")
        end
      end
    end
    count
  end
else
  def count_ruby_files
    count = 0
    queue = [File.expand_path(__dir__)]
    while dir = queue.pop
      Dir.each_child(dir) do |name|
        next if name.start_with?(".")

        abspath = File.join(dir, name)
        if File.directory?(abspath)
          queue << abspath
        else
          count += 1 if name.end_with?(".rb")
        end
      end
    end
    count
  end
end

10.times do
  count_ruby_files
end
```