`ir::Builder::add_group()` should take a done condition

[calebmkim]

trafficstars

Since we know that every group must have a group[done] = ... assignment, the ir::Builder::add_group() function should take arguments for the group's done condition.

In fact, it may even be useful to have add_group() take a Vec<ir::Assignment> argument for the group's assignments, although there may be drawbacks to this that I'm not thinking of.

[rachitnigam]

We should handle #864 when we fix this