Add stateful sets.

[tmccombs]

Currently there are pure sets, and there are set interfaces defined in lil/interface/set that say they are for both pure and stateful, but it doesn't look like there are any stateful sets defined.

[fare]

Sorry, Thayne, but I'm not actively adding features at this moment.
I can review your code and grant you commit bits, if you want.
I shouldn't be too hard to add a stateful implementation based on hash-table,
one based on bitmaps, and/or one based on automatically wrapping a pure implementation.