cleanupnodes: pass multiple predecessors to `createmarkers` directly

obsolete: allow multiple predecessors in createmarkers The logic for this change is similar to the change to `cleanupnodes` that we did earlier. Now that the rebase code is trying to record a fold, we need to actually record it in the markers. The first step is to have the markers creation API able to receive such fold data. To keep things sane, we restrict fold to on successors.

obsolete: preindent code in createmarkers This will help to make the next change clearer.

obsolete: explicitly pass relation items to effectflag computation To track folds, we are about to change the content of `rel`. To simplify this change, we update the `geteffectflag` function beforehand.

rebase: explicitly track collapses as fold A collapse is a large fold. It is now explicitly tracked (at the cleanupnodes API level).

rebase: use tuple as `replacement` keys Now that `cleanupnodes` support tuples as key, we update the rebase code to use them. No changes in the replacement tracked are introduced yet.