Speed up attribute lookup

[preciz]

Improvements:
exact attribute => ~5% faster, ~50% lower memory usage
attribute present => ~10% faster, ~40% lower memory usage
attribute includes => ~100% faster, ~60% lower memory usage

This change utilizes more built in functions instead of Enum and uses String.contains? to check for match before performing String.split.

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

  inputs = %{
    "big" => read_file.("big.html")
  }

  Benchee.run(
    %{
      "exact attribute" => fn doc -> Floki.find(doc, "[class='noprint']") end,
      "attribute present" => fn doc -> Floki.find(doc, "[title]") end,
      "attribute includes" => fn doc -> Floki.find(doc, "[class~='wikitable']") end
    },
    time: 10,
    inputs: inputs,
    memory_time: 2
  )

[philss]

How much of the improvements came from this change?

If is not that much, I would prefer to keep the Enum.find_value/3 just because it's easier to read and maintain.

[preciz]

I just ran the benchmarks back and forth changing this part only and it's faster in all cases, the biggest win in speedup is that it's ~11% faster in the "exact_attribute" case but most significantly it halves the memory usage in the "exact_attribute" and in the "attribute_includes" cases.

I believe this library is used heavily by a lot of companies where every speedup has a huge effect on throughput.
That is the case for our company.