[5.2] Add a method to map keys

[phroggyy]

This PR introduces mapKeys to the Collection, to allow you to modify keys rather than values. A prime use case for this is in https://github.com/laravel/framework/blob/5.2/src/Illuminate/Database/Eloquent/Relations/MorphTo.php#L197-L205 where that code could be replaced by

return BaseCollection::make($this->query->getEagerLoads())
    ->filter(function ($constraint, $relation) {
        return Str::startsWith($relation, $this->relation.'.');
    })->mapKeys(function ($key) {
        return Str::replaceFirst($this->relation.'.', '', $key);
    })->merge($instance->getEagerLoads())->all();

In this PR, I also decided to do just that, because why not. This also makes #13741 unnecessary.

[phroggyy]

@taylorotwell any chance we can merge this? It just makes things clearer and ensures people don't have to do workarounds to edit keys.

[JosephSilber]

For the record, this can be achieved with the keyBy method.