Use fast drupal static pattern for locale

more discuss: #1135950: Remove static caching in t()

This patch is an exact copy of what is done in user_access()

  // Use the advanced drupal_static() pattern, since this is called very often.
  static $drupal_static_fast;
  if (!isset($drupal_static_fast)) {
    $drupal_static_fast['perm'] = &drupal_static(__FUNCTION__);
  }
  $perm = &$drupal_static_fast['perm'];

So as this is in core I'd say code wise it is good.

On the performance side I did test this on our biggest site and added some xhprof screenshots as well. It's over 2.500 calls to drupal_static() we can cut away with this in my environment.

Standard Drupal Core

Patched Drupal Core

Looks good, thanks for the xhprof screenshots :)

Committed/pushed to 8.x.

Moving for backport just in case.

Very same patch as in D8 except the file path. So I set this again to RTBC.

Fancy pants. :)

Committed and pushed to 7.x. Thanks!