Add a tombstoneless OR-Set (ORSWOT) and a Map backed by it

GC looked like too much trouble, so with a bit of thought we came up with a tombstoneless set. Turns out it is just a state based, DVV backed version of this http://arxiv.org/abs/1210.3368. But it was nice to arrive there by our own route and we all understand both DVV and ORSWOT way better now.

I'll gladly squash the commits down after review.