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

This is related to https://github.com/rust-lang/rust-clippy/issues/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.

Start by reading the related Clippy issue #4377 and the examples of dangerous blocking calls described here, including block_on and .wait() in async contexts. Determine the supported construct set and the expected lint behavior before identifying the relevant Clippy implementation and test locations. Done means the agreed dangerous constructs are detected without treating general blocking operations as in scope.