id_hash() was not at all working as expected for very small bucket size (when (size / 4) was zero). Simplified to be trivially correct.
Also added a used flag on adding an entry, which is set by callers if they have that association in active use. Those get priority over unused entries on collision.
Fleshed out test to be at least elementary useful.
Each group chat now uses an own, small assoc (80 entries).
