modes:mod:Hand: is_proper and is_improper indicator functions

A "proper" hand is one that has no wild cards - an "improper" hand is
one that does have at least one wild card.

~is_proper~ should be implemented for any Hand - ~is_improper~ falls
out for free.

Implemented for Single and Pair.

src/modes/mod.rs

@@ -10,8 +10,16 @@ pub enum Footstool {
 
 pub trait Hand {
     /** Given two instances of a Hand (`self` and `other`), verify if `self`
-    footstools `other`. */
+    footstools `other`.
+     */
     fn footstool(&self, other: &Self) -> Footstool;
+
+    // Only need to implement is_proper.
+    fn is_proper(&self) -> bool;
+
+    fn is_improper(&self) -> bool {
+        !self.is_proper()
+    }
 }
 
 mod tests {

src/modes/pair.rs

@@ -43,6 +43,10 @@ use crate::modes::single::Single;
 use crate::modes::{Footstool, Hand};
 
 impl Hand for Pair {
+    fn is_proper(&self) -> bool {
+        matches!(self.0, Card::PlayingCard(_))
+    }
+
     fn footstool(&self, other: &Self) -> Footstool {
         // A pair footstools the other <=> the highest cards of both footstool
         // each other => we can rely on the footstool implementation of Single

src/modes/single.rs

@@ -18,6 +18,11 @@ impl Single {
 use crate::modes::{Footstool, Hand};
 
 impl Hand for Single {
+    fn is_proper(&self) -> bool {
+        // Always true as Jokers are not allowed
+        true
+    }
+
     fn footstool(&self, other: &Self) -> Footstool {
         let self_abs = self.0.deck_abs();
         let other_abs = other.0.deck_abs();