Check that IO runtime != CPU runtime #174
Description
What
Add a check that makes sure that calls to async host methods and to async_in_sync_context are never done from the tokio I/O runtime.
Why
With #173 it becomes somewhat important that the I/O runtime stays responsive. In general it's somewhat a footgun if people mix these two runtimes, even without #173.
How
The only way to reason about the identity of a tokio runtime seems to be tokio::runtime::Handle::id -- which is currently marked as "unstable". Hence we need to wait til it is available without compiling tokio with tokio_unstable.