Default {} is confusing, while {:#} is much better

We've been using anyhow extensively, and it's worked out great. There's just one super confusing thing. I would expect:

```
println!("{}", anyhow!("My error message").context("on a file"))
```

To print at least `My error message`, and ideally `on a file`. But it just prints `on a file`. I know `{:#}` is available, but I've yet to find a case where we didn't want `{:#}` and the usage of `{}` is pretty pervasive - we've had it when printing messages nested, as format strings to `thiserror` etc. It is the bug we seem to make every single time. I appreciate compatibility constraints might be hard, but switching the two (or even making them both the same) would be easier to use.