Map output of concurrecy calls to the index of the input

[ovp87]

Changes the output of the concurrency calls in the Process and Fork drivers, to respect the index on the input.
This was already being done when using the sync driver, this brings the output of the other drivers into sync.

Meaning that this is now possible regardless of driver:

$result = Concurrency::run([
    'task1' => function () {
        return 'Result of task 1';
    },
    'task2' => function () {
        return 'Result of task 2';
    },
]);

With an output that matches the index of the input, essentially allowing you to index the output from each of the calls:

['task1' => 'Result of task 1', 'task2' => 'Result of task 2']

I found this useful when dealing with a dynamic number of closures, where to use previous mapping solutions
[$first, $second] = Concurrency::run(...) was tricky.

[taylorotwell]

Hey there @ovp87 - could you take a stab at just updating the Fork driver and I'll give it a test on my end? Please mark as ready for review when the requested changes have been made.

[ovp87]

@taylorotwell I've modified the Fork driver, and tested it, looks like it works fine.

I've commented out the test tough, since the package is not there by default, and i'm not sure if i should be adding it to the dev dependencies just for the test.