Find "dangerous" blocking constructs in async contexts (`.wait()`, `block_on`, etc.)

[emk]

This is related to #4377, which proposed warning about blocking operations like println! inside an async context. That is definitely a promising idea, but it's also very ambitious, because almost anything could block.

But there's also a specific set of blocking constructs that can potentially deadlock the entire tokio (or other) runtime. These include things like calling block_on or .wait inside an async context. There are easy to include by mistake when converting non-async code to async. And there are fewer of these dangerous calls, which would make them easier to lint for.

[michaelsproul]

Hi @emk! I've built a prototype of this lint which I've been running on CI for the last few months: #9857.

I'm looking for some help to get it to a merge-ready state