Simplify views, eliminate _as_set() method.
Simplify views, eliminate _as_set() method.

This has the added advantage of making Python 3 conversion work better by not
relying on iteritems. iteritems() becomes items(), which returns a view, making
a circular dependency. I'm not sure there are any circumstances where this would
actually break, but it's ugly.

KevinGoodsell/caduceus