It's a bit of a shitshow because of lifetimes, and the
fact that async functions can't be used as function
pointers (because the return value is not sized
predictably in a dynamic context).

This thread was really hepful to me:
https://users.rust-lang.org/t/how-to-store-async-function-pointer/38343/4

All that's left to do is:
* Fix the doctests
* Actually try out the examples and see if they are
  still working/performant.
* Go hunting for places where explicit threads are used
  which could be turned into tasks.