Portability for protocol implementations

[tmandry]

This is a roadmap item for wg-async. You can view the roadmap here: wg-async roadmap

Enable libraries like Hyper and Quinn to be executor-agnostic and easy to use with any runtime.

Dependencies:

• RFC: I/O traits (async Read/Write) #282
• RFC: Task spawn trait #283
• RFC: Timer trait #284

[Noah-Kennedy]

Could you add a description for this issue?