Add type generics to iterator_to_array and SeekableIterator

[yottalogical]

This adds type template generics to iterator_to_array and SeekableIterator. This allows static analyzers to automatically determine the key and value types of the array returned by iterator_to_array, such as in this example:

<?php

/** @var SeekableIterator<int, string> */
$iterator = new ArrayIterator(['a', 'b', 'c']);

$array = iterator_to_array($iterator);

// Static analyzers know the types of $key and $value
foreach ($array as $key => $value) {
	echo "$key: $value" . PHP_EOL;
}