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/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();