Using the persistent nodemap require a compeling performance boost and an
existing implementation. The benefit of the persistent nodemap for pure python
code is unclear and we don't have a C implementation for it. Yet we would like
to actually start testing it in more details and define an API for using that
We introduce a new devel config option to use an index class dedicated to
Nodemap Testing. This feature is "pure" only because having using a pure-python
index with the cext policy proved more difficult than I would like.
There is nothing going on in that class for now, but the coming changeset will