[Async-sig] Inadvertent layering of synchronous code as frameworks adopt asyncio

[Glyph]

> There's another form of nested event loop invocation where two separate event loop objects exist. That is a much more worrisome scenario, because callbacks associated with one event loop won't run at all while one is waiting for a task on the other loop.

This strikes me as a much *less* worrisome scenario. It’s probably a bad idea in application code (just ... await the thing you want to run_until_complete?) but it allows things like debuggers and performance telemetry reporters to use asyncio internally while presenting a necessarily synchronous interface to the caller.

So this might be a huge performance problem if you do it accidentally, but it’ll also be relatively easy to spot, and crucially it doesn’t violate any explicit guarantees of the underlying API.
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