stablerange: introduce caching for the full revision in a set Such cache proved handy in the "per-range" class so we carry it along to the unified class. cf documentation for details.

stablerange: add a cache for stablesort ordering This will be very handy for merge, cf inline documentation for details.

stablerange: move revs computation within the main class We still need to compute the revision withing a range when we slice a merge. This is the last large logic that remains in the individual class and we migrate is on the main class.

stablerange: minor method reorders on the main class We put the public method first for clarify.

stablerange: drop "key" and "id" logic form the class We can restrict to the bare minimun for equality and hashing now.

stablerange: drop length from the class There is not remaining user.

stablerange: drop _depth Nothing uses it anymore.