Add Arr::keyMap() method

[Muetze42]

Run a map over each of the keys in the array.

$array = [
    'Foo' => 1,
    'Bar' => 2,
];

return Arr::keyMap($array, 'strtolower');

Result:

$array = [
    'foo' => 1,
    'bar' => 2,
];

[taylorotwell]

You can just use mapWithKeys.