WIP: futures-await

Syntactic sugar and generators, HUURAY!

We currently await(upstream):

• generators stable Rust
• conservative_impl_trait stable
• proc_macro stable
• syntactic sugar for streams in futures-await, whenever the sugar is agreed upon.
• futures-await to solve borrowed parameters

We should do ourselves:

• Re-enable stable/beta in Gitlab CI
• Do something whenever futures-await solves borrowed parameters
• Swap streams with syntactic sugar, whenever the sugar is agreed upon.