stablesort: introduce a cache object The object is not caching anything yet, but the API are getting into place.

stablesort: start implementing more advanced version of headstart sorting Now that we can validate the sort with a basic implementation, we can start implementing a version that will support caching and other goodies.

stablerange: make use of the limit argument in 'mergepoint' This will eventually avoid doing full iteration on ancestors of the head when retrieving the range content.

stablerange: introduce a smarte version of stablerange for 'mergepoint' This implementation will using caching and smarter algorithm based on these cache. For now it inherit most of it code from the dummy version for convenience. The inheritance to the dummy version should be dropped eventually.