add map example to toUpper

Give a not-so-obvious example of how to use toUpper to create a non-allocating algorithm.
Why not a documented `unittest` instead duplicating the code in the `Example`? That's precisely what we introduced the feature for.

And why `@trusted`? Every occurrence of `@trusted` is something that needs review. Just make it `@system`, there is no need for unit tests to be `@safe`.