Optimize toolbar to only build if it will be displayed

Here's the patch. In running benchmarks, I also discovered that in a situation where the anonymous user has access to the toolbar (admittedly, unlikely), warnings are triggered when building the "Hello USERNAME" link. This patch solves that by building a different link, more appropriate to an anonymous user.

The benchmark I ran was a fresh install of HEAD with the default profile, adding a single node of the Page type (title = "Title", body = "Body"), and enabling all permissions for the anonymous user. I also created and enabled a dummy module whose only code is:

function MODULE_page_alter(&$page) {
  $page['page_top']['toolbar']['#access'] = FALSE;
}

With this setup, on my computer (running MAMP, so not a production server), running "ab -c1 -n100 http://localhost:8888/d7/node/1", I got:

HEAD + just the last 2 hunks of this patch (needed to turn off PHP notices): 93.0 +/- 2.1
HEAD + full patch: 89.0 +/- 2.2