Consider making value::Stream constructor public

[KodrAus]

For proper streaming scenarios like tokio-trace where implementors of its Subscriber trait will want to hold on to a stream across an arbitrary number of calls.

It means callers would then be able to call Value::stream(1, &mut value::Stream::new(stream)). This might also be confusing within Value implementations. It needs some thought.

[KodrAus]

Alternatively, we could introduce an value::OwnedStream that wraps up a generic T: stream::Stream and debug stack, and can be borrowed as a value::Stream.

The current value::Stream expects to be borrowed, so it's not useful for scenarios where you need to own the stream across calls, and having an owned variant we can borrow from avoids having to introduce a branch into every streaming call for a Cow-like approach that bungs an owned or borrowed stream into the same structure.