[Roman Numerals] add practice exercise

[colinleach]

The performance stuff for reverse-string is going badly, so I did this in the interim. It's for week [02-06]. Only the exercise, no Deep Dive stuff.

It works, but the example implementation may not be the most idiomatic R. A while loop inside a for loop is standard across most languages (except the ones relying heavily on pattern matching), but I'll be very happy to be shown that I'm missing something R-specific.

Feel free to replace with something better if you wish: we can use all the implementations for approaches later. It certainly isn't vectorized, but I couldn't see how to make that work within the function.

[colinleach]

Scrap this. I'm not in best health (again) today, and failed to create a separate branch. One of those days when I seem to have forgotten how to code in any language, despite doing it for nearly 50 years...