Modification of the message via format string?

Another great feature would be to be able to modify the output ala:
```
try spew.dump("Value of m = {any}", m);
```
would output:
```
Value of m = kitchensink.main.Money { Amount: u32 1000, Currency: kitchensink.main.Currency { Name: []const u8 "Euro", Symbol: [const u8 "€" } }
```