Add named unique() "bags"

[haggys22]

I want to seed two table columns independently from each other but with the same set of possible values, e.g. [1, 2, 3, 4, 5, 6, 7, 8]

My Laravel factory and seeder look like this:

// Factory
$factory->define(MyModel::class, function (Faker $faker) {
    return [
        'a' => $faker->unique()->randomElement([1, 2, 3, 4, 5, 6, 7, 8]),
        'b' => $faker->unique()->randomElement([1, 2, 3, 4, 5, 6, 7, 8])
    ];
});
// Seeder
public function run() {
    factory(MyModel::class, 4)->create();
}

Currently, the UniqueGenerator will save the existing values only by function name, e.g. "randomElement". Therefore I can only seed up to 4 models and receive no a value as a b value.

Solution:
With this pull request I propose adding a $bag parameter to the unique() modifier to define independent sets of existing values. The given example factory could then be changed to

$factory->define(MyModel::class, function (Faker $faker) {
    return [
        'a' => $faker->unique('a')->randomElement([1, 2, 3, 4, 5, 6, 7, 8]),
        'b' => $faker->unique('b')->randomElement([1, 2, 3, 4, 5, 6, 7, 8])
    ];
});

This allows generation of up to 8 models with distinct values per column (a and b) but same values using the same formatter for different columns.

When used without the $bag parameter the behavior will not change.

[pimjansen]

Please see #1851