[data] Chunk improvements (#962)

- Extend `StrictOptimizedSeqOps` with `Chunk` to optimize `Seq` based
methods
- Improve APIs to correctly return `Chunk[A]` instead of `Seq[A]`
- Optimize `Chunk.Indexed#iterator`
- Optimize `Chunk#take` to avoid allocations
- Optimize `Chunk#appended` to use `Chunk#append`