stream: introduce Body

This introduces a new stream primitive called Body which helps with performance, ergonomics and compatibility when working with different types of data producers and consumers.

Using Body it will be possible to delay converting streamlike objects as long as possible and enable some optimizations where we can avoid e.g. intermediate node streams.

The current implementation does not yet take advantage of possible optimizations. However, I think that is something we can add and improve on in the future.