Add support for optional mapper in toSet() and toList()

[thekid]

This makes these two methods consistent with toMap(). This comes in handy when using Collectors::toX() in conjunction with grouping:

$namesByDepartment= Sequence::of($this->employees)
  ->collect(Collectors::groupingBy(
    function($e) { return $e->department()->name(); },
    Collectors::toList(function($e) { return $e->name(); })
  ))
;

// = [
//  'System Administration' => ['Jon', 'Paul'],
//  'Infrastructure Development' => ['Ben', 'The Dude']
// ]