[5.8] Add countBy method to Collection

[jamierumbelow]

Adds a countBy method to Illuminate\Support\Collection.

By default, this method counts each appearance of a given value:

collect([ 1, 2, 2, 2, 3 ])->countBy(); // == collect([ 1 => 1, 2 => 3, 3 => 1 ])

It also accepts a predicate:

collect([ 1, 2, 2, 3, 4 ])->countBy(function ($i) {
    return $i % 2 === 0;
}); // == collect([ true => 3, false => 2 ])

collect([ 'alice@gmail.com', 'bob@yahoo.fr', 'carlos@gmail.com' ])
    ->countBy(function ($email) {
        return substr(strrchr($email, "@"), 1);
    }); // == collect([ 'gmail.com' => 2, 'yahoo.fr' => 1 ])

[driesvints]

Please fix the StyleCI warnings.

[BrandonSurowiec]

Suggested change

	$predicate = function ($val, $key) {
	$predicate = function ($val) {

You aren't utilizing the $key so it doesn't need to be set.

StyleCI warnings fixed.

[vlakoff]

For the "without predicate" mode, couldn't we use array_count_values()? Probably much, much faster.