Support futures / async-std traits

By supporting the traits from `futures`, library users who use async-std (and a few other runtimes that use traits from `futures`) wouldn't need to pull in Tokio's competing set of async traits. Constantly creating compatibility layers between the Tokio traits and the `futures` traits becomes tedious after a while.

The issue names no files, tests, or entry points. Start by tracing the library's current Tokio trait integration and dependency setup; done means users of async-std and other futures-based runtimes can use the library without Tokio trait compatibility layers.

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