Streams/Channels documentation for the async-std book

[ragona]

Hi friends,

This is a first draft of a quick helper section for Streams. I've actually only used Streams in combination with channels, and I wanted to see if I could introduce them independently. I think it's a little odd (and not very concurrent!), and would love any feedback. I'm also not familiar with all the helper methods in stream so I would love help adding notes there.

Here are the outstanding items:

• [ ] Add streams (Draft)
• [ ] Add channels
• [ ] Add streams and channels
• [ ] Add links to documentation.
• [ ] Add notes on things like stream::from_fn and all the other awesome helper methods.

Hope you're having a great weekend!

Best, Ryan