Micro optimization in _registry_update

This piece fo code in registry.inc is terribly inefficient calling array_keys for (the relatively large array) $files in every loop iteration.

foreach ($lookup_cache as $key => $file) {
  // If the file for this cached resource is carried over unchanged from
  // the last registry build, then we can safely re-cache it.
  if ($file && in_array($file, array_keys($files)) && !in_array($file, $parsed_files)) {
    $unchanged_resources[$key] = $file; 
  }
}

On my machine it takes abount 20 miliseconds to run this loop. Using array_key_exists instead cuts the execution time to 0.1 miliseconds, making it two orders of magnitude faster.

foreach ($lookup_cache as $key => $file) {
  // If the file for this cached resource is carried over unchanged from
  // the last registry build, then we can safely re-cache it.
  if ($file && array_key_exists($file, $files) && !in_array($file, $parsed_files)) {
    $unchanged_resources[$key] = $file; 
  }
}

I know this is nitpicking as this code is not run very often, but such it's such a small, 100% safe, change (with a marginally positive effect on performance) i still think it could be worth committing.