Default panic message should print Box<dyn Any>
Default panic message should print Box<dyn Any>

Prior to this patch, the default panic message (resulting from calling
`panic_any(42);` for example), would print the following error message:

```
thread 'main' panicked at 'Box<Any>', ...
```

However, this should be `Box<dyn Any>` instead.

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust