Debug output is broken for dynamic matrices

In #986 the `Debug` formatting of matrices was changed to forward to the storage. This works reasonably well for static matrices, but unfortunately it does not work well for dynamic matrices.

Given this code ([Playground link](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=3e2232974ea698a3e018e66d99530642) ):

```rust
let a = dmatrix![1.0, 2.0; 3.0, 4.0];
println!("{:#?}", a);
```
the output is
```
VecStorage {
data: [
1.0,
3.0,
2.0,
4.0,
],
nrows: Dynamic {
value: 2,
},
ncols: Dynamic {
value: 2,
},
}
```
Clearly, we don't want a matrix to be represented *directly* as a "VecStorage". I think the "fix" here is to go with the proposal in #898 and apply it uniformly across static or dynamic matrices.