Moved a bunch of imports in the defaultfilters library out of functions. On…

Moved a bunch of imports in the defaultfilters library out of functions.  On CPython this is actually a big performance win if one of these filters is your bottleneck.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16283 bcc190cf-cafb-0310-a4f2-bffc1f526a37