Tokio has `Stream` support in tokio-stream for stability concerns

[nikomatsakis]

Brief summary

Tokio has support for the futures Stream trait but it needs to be loaded from the tokio-stream crate because of stability concerns (they want to give a strong API stability guarantee, but the futures crate doesn't have a published guarantee, so they don't want a direct dependency). This affects some APIs such as the TcpListener.

On a related note, async-std embeds the futures traits directly, which makes a futures 1.0 release have potential impact on their public facing APIs and semver (see e.g. https://github.com/rust-lang/futures-rs/issues/2362).

Optional details

• (Optional) Which character(s) would be the best fit and why?
  • [ ] Alan: the experienced "GC'd language" developer, new to Rust
  • [ ] Grace: the systems programming expert, new to Rust
  • [ ] Niklaus: new programmer from an unconventional background
  • [ ] Barbara: the experienced Rust developer
• (Optional) Which project(s) would be the best fit and why?
  • List some projects here.
• (Optional) What are the key points or morals to emphasize?
  • Write some morals here.

[zeenix]

Oh, perhaps we should have covered this as part of #176?