`Any` pitfalls

Since any value can be converted to an Any trait object, you can run into subtle bugs where you put the wrong value behind the Any:

use std::any::Any;

struct Foo;

fn main() {
    let x: Box<Foo> = Box::new(Foo);
    let mut y: Box<Any> = x;
    let z: &mut Any = &mut *y; // remove the `*` and the next line panics
    let a: &mut Foo = z.downcast_mut().unwrap(); // panics if `z` contains `Box<Any>` instead of `Any`
}

We should be linting all coercions of references/boxes to Any to a reference to Any. Especially around double references and similar this seems like it can go south very fast

Start by reproducing the supplied Rust example and comparing the reference and box coercion cases described in the issue. Done means the lint identifies the risky conversions to Any, including double-reference cases, with its supported cases and expected diagnostics covered by tests.