`Dyn` could have `DimName` implemented upstream

I'd like to be able to do something to support both static and dynamic allocation on one generic type like below,
```rs
impl ::rand::distributions::Distribution for Mut {
fn sample(...) -> DVector { ... }
}
```

```rs
impl ::rand::distributions::Distribution> for Dirichlet
where
D: Dim + DimName,
nalgebra::DefaultAllocator: nalgebra::allocator::Allocator,
{
fn sample(...) -> OVector { ... }
}
```

but the interface has a problem the second `impl` could duplicate the first if `Dyn` implements `DimName` upstream.

Am I going about this the wrong way, or can I request to seal the `DimName` trait against being implemented on other structs?