"no recursion" check will not work for async fns/generators in traits

We currently check that async fns and generators are not recursive, as this would generate infinitely large state machines. However, if we ever want to support async fns or generators in traits, this check would have to run post-monomorphisation (since until that point we don't know which trait implementation will be chosen).

Since we don't like to generate errors post-monomorphisation, this seems like a serious problem!

Example:

trait Foo {
    async fn foo(&self);
}

impl Foo for i32 {
    async fn foo(&self, other: &impl Foo) {
        other.foo().await;
    }
}

impl Foo for i64 {
    async fn foo(&self, other: &impl Foo) {
        // Not recursive
    }
}

fn main() {
    let x = 1i32;
    x.foo(&x); // Error
}

Start with the issue's async-trait example and review how the existing no-recursion check is performed for async functions and generators. Trace where trait implementation selection and monomorphisation occur, then determine a design that detects recursive state-machine generation without introducing post-monomorphisation errors.