fix `LazyMap` to avoid unnecessary function calls when not accessing items in order (fixes http://dev.plone.org/plone/ticket/9018)

Moved subset id calculation in `OFS.OrderSupport.moveObjectsByDelta` to a new helper method, patch by Tom Gross. This allows CMFPlone and Archetypes to re-use the OFS implementation instead of using their own copy of the modified Zope 2.7 codebase :)

svn+ssh://svn.zope.org/repos/main/Zope/branches/2.12 r118262:118470

Fixes LP #628448

Avoid conflict error hotspot in PluginIndexes' Unindex class by using IITreeSets instead of simple ints from the start. Idea taken from ``enfold.fixes``. This optimizes the common case of FieldIndexes being used for a low number of unique values. The edge-case of having a FieldIndex for an unique value per document gets penalized, as it uses many more persistent objects. This really warrants a separate type of index.