Rewrite `setEntity` function

[rmrhz]

Rewrite the setEntity function to accomodate certain functionalities that will improve usability cases

Improvements

• [x] Must persists to $this->entity
• [x] Must return as \Prjkt\Component\Repofuck\Repofuck
• [x] Use entity() function instead of providing an object from the $entity parameter

Possible Future Use Cases

$keys = ['foo', 'bar'];

$foo->update($request->all(), $keys, function($repo)
{
    // assuming that from the request `bar` is an array
    $bar = $repo->setEntity('bar')->create($repo->data['bar'], array_keys($repo->data['bar']));
    $repo->pluckKeys(['bar'])->pushKeys(['bar_id'])->pushData(['bar_id' => $bar->id]);
});