Migrated regressiontest/datastructures doctest, and moved it into the existing...

Migrated regressiontest/datastructures doctest, and moved it into the existing utils datastructures package. Thanks to Stephan Jaekel.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13936 bcc190cf-cafb-0310-a4f2-bffc1f526a37