generatorset: explicitly track iteration order The expected iteration order may be different than the fast iteration order (eg: ancestors(42) is expected to be iterated upward but is fast/lazy to compute downward. So we explicitly track the iteration order and enforce it if the manual iteration is requested. Default expected iteration order of a generator set is ascending because I'm not aware of any descending revset that need a generatorset. The first to find such descending revset will have the pleasure to make this configurable.

addset: drop caching through generatorset The utility of this cache is debatable (no visible benchmark impact) and using generatorset for such purpose makes the code complicated. We drop it for now. Someone can reintroduce a smart version of it in the future if it is detected to be relevant.

generatorset: get list-based fast iterations after the generator is consumed When all revisions are known, we shortcut most of the class logic to use list iteration instead. The cost of the sort is expected to be non-significant. The list creation and sorting could be done lazily in the future. We have to copy the list to not break existing iterator created before we finished consuming the generator.

generatorset: move iteration code into _iterator _iterator handles the generator iteration. The `__iter__` method will need changes to handle ordering-related information.