Use trait instead of implement for transformer binding

[Propaganistas]

Hi

Just an idea: transformers currently have 2 ways to be injected:

1. using a dedicated class and an API::transform call
2. by implementing Dingo\Api\Transformer\TransformableInterface in the model

In my opinion the second option is more clean as I this seems to me like some sort of model definition similar to getAttribute().
However, how about defining the implementation as a trait? This avoids collision when a model needs multiple interfaces. (Actually I think packages never should define interfaces for models just for this reason)

Some code suggestions:

Change Dingo\API\Transformer\TransformableInterface to Dingo\API\Transformer\TransformableTrait:

trait TransformableTrait {
    /**
     * Get the transformer instance.
     * Should be overridden in the implementing class.
     * 
     * @return mixed
     */
    public function getTransformer() {
        return array();
    }
}

In Dingo\API\Transformer\Factory:

/**
 * Determine if the class is bound by the transformable contract.
 *
 * @param  string|object  $class
 * @return bool
 */
protected function boundByContract($class)
{
    return is_object($class) and in_array('TransformableTrait', class_uses($class));
}

[jasonlewis]

A class can implement many interfaces so it should be a non-issue.

class Example implements FooInterface, BarInterface {

}

[Propaganistas]

Oh god I'm ashamed; confusing extends with implements...
This is indeed a non-issue.

[jasonlewis]

Blame it on lack of sleep. 😄