Since we want to look up items by pointer, just store them in a
std::unordered_set, which allows fast find().
This dramatically simplifies most operations on this data structure.

Fix a dodgy sd test that was relying on items with the same whichid
being in the pool being in a certain order.

Change-Id: I4d79fc718f95e3083a20788be1050fbe9fca7263
Reviewed-on: https://gerrit.libreoffice.org/70881
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>