Wrapping C++ async APIs in Rust futures

[pcwalton]

• Brief summary: Grace wants to wrap C++ async APIs using the C++23 executor API in Rust futures and use them from a Tokio app.
• Character: Grace
• Key points or morals (if known):
  Cancellation is the hard part here, because dropping a C++ task is undefined behavior while in Rust this is idiomatic. C++ tasks can close over non-owning references that are expected not to go away while the operation is in flight, so this is important. We want to make C++ async APIs easy to use with as little runtime overhead as possible.

[nikomatsakis]

@pcwalton and I had a good chat about this, we took notes in this hackmd. I would like to turn it into a status quo story.