stablerange: stop using '.node' in subrangesclosure We access the range-head revision number by index and convert it on site.

stablerange: have the class behave as a tuple If we want people to use a tuple, we should offer them a tuple. This should help update most of the code iteratively.

obshashrange: extract computation back into the discovery module This obshash is related to discovery and it seems more appropriate to have to live there. This remove the last large piece of logic from the class. We'll now be able to slowly turn it into a tuple.