lib/container: Improve efficiency.
lib/container: Improve efficiency.

From the DictMixin documentation, __contains__(), __iter__(), and iteritems()
all improve the efficiency of things, so make sure they're all implemented.

MostAwesomeDude/construct,0000-bigtree/construct,MostAwesomeDude/construct,mosquito/construct,mosquito/construct,gkonstantyno/construct,riggs/construct,0000-bigtree/construct,gkonstantyno/construct,riggs/construct