stablesort: introduce a mergepoint based method focused on head The things we needs for stable range is head centric. So we simplify the issue by focussing on head.

stablesort: introduce a "mergepoint" method The mergepoint method decide order from merge. This allow to directly reuse a the full sort of one of the parent, and should simplify caching.

stablerange: fallback to a more naive approach to find subrange As suspected, using "bheads" was bit good enough in some case and we have to fall back to the approach used in "basic". On the evolve repo (about 3K changeset) this approach is about 60% slower than the previous (wrong) code.

stablebranch: avoid overlap between subrange Subrange overlap make things more complicated and less efficient. We fix the computation done in the "cached" version of the stablerange cache. This bring things in line with the result produced by the basic version. Since we already bumped the format version since the last release, I don't things we need to do anything else.

stablerange: introduce an 'basicstablerange' This class hold all the basic of stable range but the part specific to the sorting. This allow to clearly express what is expected from each method and to generate tests output to be validated against the more complex implementation. While writing this, we spotted a bug in the current stable range implementation. Fix coming.

stablerange: extract the core API into a 'stablerangecore' class To clarify what the code is doing without all the caching and algorithmic improvement, we extract a core class that highlight the basic API expected from the class.