revset: turn spanset into a factory function...
revset: turn spanset into a factory function
We rename the `spanset` class to `_spanset`. `spanset` is now a function that
builds either a `fullreposet` or a `_spanset` according to the argument passed.
At some point, we may force people to explicitly use the `fullreposet`
constructor, but the current approach makes it easier to ensure we use the new
class whenever possible and focus on the benefits of this class.