modes:single: implement Hand for Singles

Really trivial implementation.

src/modes/single.rs

@@ -9,6 +9,24 @@ impl Single {
     }
 }
 
+use crate::helper::ordered;
+use crate::modes::{Footstool, Hand};
+
+impl Hand for Single {
+    fn footstool(&self, other: Self) -> Footstool {
+        let self_abs = self.0.deck_abs();
+        let other_abs = other.0.deck_abs();
+
+        if self_abs == other_abs {
+            Footstool::Full
+        } else if self_abs == other_abs + 1 {
+            Footstool::Half
+        } else {
+            Footstool::None
+        }
+    }
+}
+
 use std::fmt::{Display, Formatter, Result};
 impl Display for Single {
     fn fmt(&self, f: &mut Formatter<'_>) -> Result {