DEP-0007 Type-Safe Limited Collections

From the DEP-0007 document (http://opendylan.org/proposals/dep-0007.html):

To improve type safety of collections and particularly limited collections, this DEP adds a ``default-fill:`` keyword argument to limited collection classes and an ``element-type-fill`` and ``element-type`` getter to all collections.

The reference implementation is now complete and tested and is ready to be accepted or reject per DEP guidelines.