Replace uses of each()

Replace uses of each()

It's deprecated in PHP 7.2, may as well replace it now.

I note that, contrary to claims at
https://wiki.php.net/rfc/deprecations_php_7_2#each, none of our uses
were trivially replaceable with foreach.

* wfArrayDiff2_cmp() is processing two arrays by value in parallel.
* MagicWordArray::parseMatch() is doing something funky with the data
  structure returned by preg_match().
* HashRing was using it like "nextKey()", replaced with calls to key()
  and next().
* FormatMetadata and IndexPager were both using it as a shorter way to
  get both key() and current() for the first element in the array. I
  suppose a foreach(){ break; } would do the same, but that's confusing.

Bug: T174354
Change-Id: I36169a04c764fdf1bfd6603395111c6fe0aae5eb