Add Opaque as shorthand for Box<dyn Any + Send>

[lu-zero]

It makes the code a little tidy and paves the way to add Sync to the traits once rust-lang/rust#80945 lands.

[coveralls]

Coverage increased (+0.3%) to 83.268% when pulling 85fc222 on rust-av:opaque-shorthand into 165b9b5 on xiph:master.

[sdroege]

It makes the code a little tidy and paves the way to add Sync to the traits once rust-lang/rust#80945 lands.

Or you do this for the time being in your code:

fn downcast<T: Any + Send + Sync>(
    b: Box<dyn Any + Send + Sync>,
) -> Result<Box<T>, Box<dyn Any + Send + Sync>> {
    if b.is::<T>() {
        unsafe {
            let raw: *mut (dyn Any + Send + Sync) = Box::into_raw(b);
            Ok(Box::from_raw(raw as *mut T))
        }
    } else {
        Err(b)
    }
}

[sdroege]

Looks good to me, but is of course an API change

[lu-zero]

I slated it for the release 0.5.0.