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.

stablerange: introduce an intermediary abstract class for caching The 'branchpoint' and 'mergepoint' based stablerange class will need similar cache, we introduce an abstract class to take care of that first.

stablerange: introduce a basic-mergepoint method This method use the new 'mergepoint' based stable sorting. It is expected to be eventually better than branch point to stable range: more stable, easier to cache, etc. Having a basic implementation will allow use to validate more advanced implementation.