Dataset helpers should return distinct values #25

At the moment DatasetWrapper.subjectsOf (and the other mwthods here) might return the same term multiple times in its result Iterable, though intuition would imply distinct values.

Consider this input:

<s> <p> <o1> .
<s> <p> <o2> .

subjectsOf("p") translates (conceptually) to the following:

for (const quad of dataset.match(undefined, p, undefined, undefined)) {
  yield quad.subject
}

which matches both statements in the input, so it returns <s> twice.

Instead we should ensure that only unique terms are returned:

const seen = new Set<Term>
for (const quad of dataset.match(undefined, p, undefined, undefined)) {
  if (seen.has(quad.subject) continue
  seen.add(quad.subject)
  yield quad.subject
}