accumulateSubjects should deduplicate URIs

[josephguillaume]

I have a situation where a statement is repeated in two different documents and the URI is therefore repeated as a subject. Currently when reverseRelations is called, the URI is included twice.
A minimal test case could use a store with statements with two graphs. Other situations may also appear where a URI is duplicated.

This is because accumulateSubjects does not perform any kind of deduplication.
https://github.com/pod-os/PodOS/blob/main/core/src/thing/accumulateSubjects.ts

Presumably the fix is for either accumulateSubjects or reverseRelations to use a Set.
The same issue likely exists in the other similar functions.

A workaround is for the user to perform deduplication themselves