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.

stablesort: add a new test to check for fully independant branches Apparently, this was not covered.