Feature Request: implement Diagnostic for &Diagnostic

[titaniumtraveler]

I know it would be a breaking change, but I think it should definitely be considered for the next major version.

Please provide a blanked impl for &D: Diagnostic

impl<D: Diagnostic> Diagnostic for &D
{
 // ... trait methods forward to `D`
}

Considering that all methods of Diagnostic take in &self, it would be trivially implementable and is a pattern extensively used by std traits like io::Write or Iterator.

I got the problem factored out of my original use case, but IMO it is still The Right Thing To Do!