Allow mappers to use NULL keys for appending to the result

[thekid]

By using an explicit null in yield we can append to the result instead.

Note: Unfortunately, we cannot distinguish yield $value and yield 0 => $value from each other at runtime, or else we would've used the short form for this behaviour, while interpreting the one with the explicit 0 as setting $result[0].

Example

$opcodes= function() {
  yield 'const'    => 1;
  yield 'add'      => 1;
  yield 'multiply' => 2;
  yield 'add'      => -1;
};
$a= (new Assertable($opcodes()))->mappedBy(function($arg, $op) { yield null => [$op, $arg]; });
$a->value; // [['const', 1], ['add', 1], ['multiply', 2], ['add', -1]]