Optimize leftpad on raw_html

[ypconstante]

Today leftpad calls String.duplicate, which ends up being significantly costly when generating a pretty raw html, since this is used on each line being generated.

This PR moves the string creation to pad_increase, instead of doing it on every call to leftpad.

##### With input big #####
Name                 ips        average  deviation         median         99th %
bench (pr)         35.87       27.88 ms    ±11.34%       27.06 ms       41.01 ms
bench              33.05       30.26 ms     ±6.39%       30.00 ms       34.48 ms

Comparison: 
bench (pr)         35.87
bench              33.05 - 1.09x slower +2.38 ms

Memory usage statistics:

Name          Memory usage
bench (pr)        12.68 MB
bench             13.83 MB - 1.09x memory usage +1.15 MB

**All measurements for memory usage were the same**

##### With input medium #####
Name                 ips        average  deviation         median         99th %
bench (pr)        133.80        7.47 ms    ±17.97%        7.08 ms       10.64 ms
bench             117.21        8.53 ms    ±16.18%        8.29 ms       11.99 ms

Comparison: 
bench (pr)        133.80
bench             117.21 - 1.14x slower +1.06 ms

Memory usage statistics:

Name          Memory usage
bench (pr)         4.14 MB
bench              4.49 MB - 1.09x memory usage +0.36 MB

**All measurements for memory usage were the same**

##### With input small #####
Name                 ips        average  deviation         median         99th %
bench (pr)        749.69        1.33 ms    ±25.91%        1.30 ms        2.17 ms
bench             648.79        1.54 ms    ±23.00%        1.46 ms        2.48 ms

Comparison: 
bench (pr)        749.69
bench             648.79 - 1.16x slower +0.21 ms

Memory usage statistics:

Name          Memory usage
bench (pr)       891.88 KB
bench            960.57 KB - 1.08x memory usage +68.69 KB

**All measurements for memory usage were the same**

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

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

Benchee.run(
  %{
    "bench" => fn doc -> Floki.raw_html(doc, pretty: true) end
  },
  time: 10,
  inputs: inputs,
  memory_time: 2
)

[philss]

@ypconstante Thank you again! :D