Resolves #52

Creates an 8x perf improvement with 800ms to run the following vs 100ms to run with the SyncUnionIterator.

const it = new UnionIterator([range(0, 2_000_000), range(0, 2_000_000)]);

const start = Date.now();

it.on('data', () => {})
.on('end', () => {
  const end = Date.now();
  console.log(end - start)
})

@RubenVerborgh @jacoscaz This is a draft and the following decisions need to be made on it:

• Should UnionIterator just be replaced with SyncUnionIterators (I think it should)
• If not, which should union use

This is a draft because I am yet to write a test suite for this