add scaffold to lib.cairo

exercises/practice/roman-numerals/src/lib.cairo

@@ -1,5 +1,20 @@
+use core::fmt::{Display, Error, Formatter};
+
 #[derive(Drop)]
-struct Roman {}
+pub struct Roman {}
+
+impl U32IntoRoman of Into<u32, Roman> {
+    #[must_use]
+    fn into(self: u32) -> Roman {
+        panic!()
+    }
+}
+
+impl RomanDisplay of Display<Roman> {
+    fn fmt(self: @Roman, ref f: Formatter) -> Result<(), Error> {
+        panic!()
+    }
+}
 
 #[cfg(test)]
 mod tests;