modes:mod: Hand trait

Any hand from any round mode has a chance of footstooling another hand
from that same mode.  This trait (Hand) allows us to implement these
semantics (as well as any other shared semantics) for each round mode.

src/modes/mod.rs

@@ -1 +1,13 @@
+mod pair;
 mod single;
+
+#[derive(Eq, Ord, PartialEq, PartialOrd, Debug, Copy, Clone)]
+pub enum Footstool {
+    None,
+    Half,
+    Full,
+}
+
+pub trait Hand {
+    fn footstool(&self, other: Self) -> Footstool;
+}