modes: new crate representing the API for different round types

Single is our first one - trivial really.  Only thing we need to check
is that the single isn't made up of a joker.

src/modes/single.rs

@@ -0,0 +1,17 @@
+use crate::card::Card;
+
+#[derive(PartialEq, Eq, PartialOrd, Ord, Debug, Copy, Clone)]
+pub struct Single(Card);
+
+impl Single {
+    fn new(c: Card) -> Option<Single> {
+        (!c.is_joker()).then_some(Single(c))
+    }
+}
+
+use std::fmt::{Display, Formatter, Result};
+impl Display for Single {
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
+        write!(f, "Single({})", self.0)
+    }
+}