Add content on crate `futures`

[hdoordt]

Content about:

• futures::lock
• futures::channel
• Stream & Sync
• AsyncRead & AsyncWrite
• macros

Best would be to contribute it directly to the restructure branch while it's still unmerged. There's a file on that branch where the content could be added: https://github.com/tweedegolf/101-rs/blob/restructure/slides/C-multitasking/ordering.md