spanset: do a single range check in __contains__ Now that `start <= end` is always true, we can simplify this function.

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.

abstractsmartset: default implementation for `ascending` and `descending` These two methods are actually silly aliases for `sort()` and `sort(reverse=True)`. So we get that aliasing at the abstractsmartset level. We will slowly phase out all the custom implementations and eventually remove any mentions of it from the code.

revert: bring back usage of `subset & ps` in `parents` Changeset 95af98616aa7 switched the order of the operand of the "&" computation to work around an issue from repo-wide spanset. The need for a workaround has been alleviated by the introduction of `fullreposet`. So we restore it to normal. The benchmark shows no significant changes as expected. We also revert the bogus test change introduced by 95af98616aa7. The order is actually important.

color: update docstring for debug option 964dd1c491ca updated the format for the --color=debug option. This updates the documentation to match this new format.