Implement a Broadcast method

[jgrund]

Follow on from #172.

When working with dynamic views in a UI, I need the ability to fork an existing stream and wire it up to n subviews. n is not a fixed number and oscillates up and down at runtime.

Some of these views will be destroyed at runtime, while others will stay intact. So, the mechanism should handle dynamic creation and removal of forks.

In #172, @vqvu suggested a broadcast method that consumes a stream returns an object with the following interface:

new() - Creates a new viewer stream that sees values emitted by the parent.
destroy() - Destroys the parent. No new viewers can be created after this.

The broadcast#new method returns a viewer which has shared back pressure like a fork, but

• They can be created dynamically, after data has started flowing.
• The source does not get destroyed even when there are no viewers.
• If there are no viewers, the source will be paused.
• It is up to the user of broadcast to destroy the source once they are done with it.

[jgrund]

Implemented something like this outside the lib, was easy to do.

This probably doesn't need to be in core.