It turns out this takes less than a millisecond, at least when the
event loop is empty, but the setup might still be useful in the
future. Run with `cargo bench -p signal-neon-futures`.
In particular, I got the terms "fulfill" and "resolve" mixed up. This
version should have the correct use of "settled", "fulfilled",
"rejected", and (in rare cases) "resolved".
Neon provides a way to expose *synchronous* JavaScript functions from
Rust. This means that if Rust wants to wait for the result of a
JavaScript promise, it can at best return a callback to continue its
work when the promise resolves. This does not naturally compose with
Rust's `async`, which works in terms of Futures.
This commit adds a new crate, signal-neon-futures, that provides
functionality for (1) wrapping JavaScript futures so they can be
awaited on in Rust, and (2) producing a JavaScript promise that wraps
a Rust future. It does so by synchronously resuming execution of the
Rust future whenever an awaited JavaScript promise is resolved.
