Explore `sequence.Ranges()` iterator api

Found interesting sequence iterator API
```
// [start, end)
for start, end := bitmap.Ranges() {
eliasFano.AddRange(start, end)
}
```
Here: https://github.com/RoaringBitmap/roaring/pull/522/changes#diff-975f81a675e3c63d3c0caa18e03f164c870c3facb571da48b057699dc540c87fR510

I see next applications: EliasFano, sequence merges, `tx.IndexRange()`
Theoretically it can greatly improve batch-processing of sequences.

But what else? Maybe even history api may look like `value, startTxNum, endTxNum := history.Ranges(key)`

Such API is `streaming-over-network-friendly`