Reduce function calls in format_date()

OK so Moshe pointed out some issues measuring function calls with xdebug profiler (since it logs function calls specifically, but not language constructs etc.), and I found out xdebug adds overhead to function calls even if just the extension itself is enabled, which messes up my numbers a bit (not all of them on every issue, but I think it does in this case...).

Did a microbenchmark of 1 million calls to format_date() (attached the script, you'll need to change the format_date_new() function signature for the various alternatives).

Patch, xdebug disabled:

nothing: 0.244761943817 seconds
function no_op(): 0.624433040619 seconds
format_date() HEAD: 62.5531749725 seconds
format_date() patch: 63.6565389633 seconds

Patch: xdebug enabled:

nothing: 0.363921880722 seconds
function no_op(): 3.37409496307 seconds
format_date() HEAD: 107.372431993 seconds
format_date() patch: 99.9250171185 seconds

Patch, but using static instead of drupal_static(), xdebug disabled:
nothing: 0.24368596077 seconds
function no_op(): 0.654839992523 seconds
format_date() HEAD: 63.271586895 seconds
format_date() patch: 58.2883307934 seconds

Patch, but using $GLOBALS['drupal_static'] instead of drupal_static(), xdebug disabled:
nothing: 0.253867149353 seconds
function no_op(): 0.62436413765 seconds
format_date() HEAD: 62.4352011681 seconds
format_date() patch: 59.19520998 seconds

So the most we can get in real terms, is about a 6% improvement within format_date() (which is going to have a very tiny impact during the request), and this is only possible by using #537724: Consider $GLOBALS['drupal_static'] instead of drupal_static(), which needs a wider discussion.

Marking as Code Needs Work but this one's probably won't fix.

forgot to attach script.

Let's won't fix for now.