Duet takes inspiration from the amazing trio library and the structured
concurrency approach to async programming that it uses. However, duet differs
from trio in two major ways:
* Instead of a full-blown implementation of asynchronous IO, duet relies on the
  Future interface for parallelism, and provides a way to run async/await
  coroutines around those Futures. This is useful if you are using an API that
  returns futures, such as RPC libraries like gRPC. The standard Future
  interface does not implement __await__ directly, so Future instances must be
  wrapped in duet.AwaitableFuture.

* duet is re-entrant. At the top level, you run async code by calling
  duet.run(foo). Inside foo suppose you call a function that has not yet been
  fully refactored to be asynchronous, but itself calls duet.run(bar). Most
  async libraries, including trio and asyncio, will raise an exception if you
  try to "re-enter" the event loop in this way, but duet allows it. We have
  found that this can simplify the process of refactoring code to be
  asynchronous because you don't have to completely separate the sync and async
  parts of your codebase all at once.