spanset: enforce the order lazily to gain `fastasc` and `fastdesc` methods Instead of having the direction of iteration enforced through the ordering of `start` and `end` attributes of spanset, we encode the iteration direction in an explicit attribute and always store start < end. The logic for sort and reverse has to be updated. The __iter__ is now based on the newly introduced `fastasc` and `fastdesc` methods. This will allow other code simplifications in the future.

abstractsmartset: document the `fastasc` and `fastdesc` attributes/methods See the in-line documentation for details. (This is the beginning of a massive overhaul of revset).

spanset: remove ascending/descending implementation We can rely on their implementation in abstractsmartset.

baseset: remove ascending/descending redefinition We can rely on the abstractsmartset implementation.