Switch the primary slot addressing method

[boferri]

currently, the primary slot addressing method is the utilisation of olo:index. the main drawback of this approach is that the index needs more or less completely rewritten when someone inserts or deletes entries from this list. a more flexible approach is to follow the adjacency list approach, i.e., to utilise the next (olo:next) and optionally the previous (olo:previous) relationship as primary slot addressing method (which are currently secondary, i.e., both properties are optional). This has the advantage that one does not really need to update the index values of the slot all the time when the ordered list was manipulated. One can also easily assign the index number when retrieving a full list.