Use recursion instead of Enum.flat_map on find_selectors

[ypconstante]

Today find_selectors and it's child functions use Enum.flat_map to interact through the nodes to find the matching ones.

This PR replaces the Enum.flat_map and Enum.map usages with recursion, removing most of the intermediate lists this methods had, and also simplifying traverse_with implementation.

This also provides a really small performance improvement. When checking this benchmark with fprof, operations count went from 65867 to 57726, reducing the time to run with fprof from 115ms to 95ms.

Name            ips        average  deviation         median         99th %
pr           555.12        1.80 ms    ±13.53%        1.72 ms        2.75 ms
today        544.31        1.84 ms    ±12.83%        1.75 ms        2.79 ms

Comparison: 
pr           555.12
today        544.31 - 1.02x slower +0.0358 ms

Memory usage statistics:

Name     Memory usage
pr            2.05 MB
today         2.09 MB - 1.02x memory usage +0.0319 MB

read_file = fn name ->
  __ENV__.file
  |> Path.dirname()
  |> Path.join(name)
  |> File.read!()
end

html_input = read_file.("small.html")

[{"html", _, _} = html | _] = Floki.parse_document!(html_input)

Benchee.run(
  %{
    "bench" => fn -> Floki.Finder.find(html, ".mw-parser-output > p")  end
  },
  time: 5,
  memory_time: 2
)

[philss]

I have to say it was hard to remember this part of the code 😅
Nicely done! Thank you! 💜