Use stack on Finder

[ypconstante]

Today Finder uses Enum.reduce and nested calls to traverse_with to do it's filtering.

This works fine when using HTMLTree, but this structure makes it harder to optimize some cases in which the HTMLTree isn't necessary (#515).

This PRs updates Finder to use a stack, similar to what we have on HTMLTree.build_tree. This makes it easier to implement the change for the ticket above, or other future changes to operations that don't need the HTMLTree.build step.

I also did some other clean ups in this module, I've split each change in a separated commit.

Performance is basically the same, but when profiling the timing is a lot better and more readable because of the removal of Enum.reduce

Before - selector span a * i

%                                                           CNT           ACC           OWN        
[{ "<0.99.0>",                                           278908,    undefined,      442.743}].   %%

{  {'Elixir.Enum','-reduce/3-lists^foldl/2-0-',3},        29692,      402.120,       50.704}.      
{  {'Elixir.Floki.Finder',traverse_with,4},               18660,      309.861,       50.072}.      
{  {'Elixir.Floki.Selector','match?',3},                  12986,      160.655,       42.984}.      
{  {'Elixir.Floki.HTMLTree',build_tree,5},                15867,       39.761,       28.100}.      
{  {'Elixir.String',split,3},                              6522,       73.182,       26.562}.      

After

%                                                           CNT           ACC           OWN        
[{ "<0.99.0>",                                           231183,    undefined,      363.851}].   %%

{  {'Elixir.Floki.Finder',traverse_with,4},               38959,      322.261,       66.273}.      
{  {'Elixir.Floki.Selector','match?',3},                  12986,      158.041,       42.423}.      
{  {'Elixir.Floki.HTMLTree',build_tree,5},                15867,       40.198,       28.321}.      
{  {'Elixir.String',split,3},                              6522,       71.817,       26.114}.       

[philss]

Makes totally sense! Really cool!