modes:single|pair: make default struct construction private

If all fields are public, users are allowed to use the default struct
constructor.  Obviously we want users to only use the static ::new
method.

src/modes/pair.rs

@@ -4,7 +4,7 @@ use crate::{
 };
 
 #[derive(Eq, Debug, Copy, Clone)]
-pub struct Pair(pub Card, pub Card);
+pub struct Pair(Card, Card);
 
 impl Pair {
     /** Create a new pair utilising two cards, `c1` and `c2`.  Will return None

src/modes/single.rs

@@ -1,7 +1,7 @@
 use crate::card::Card;
 
 #[derive(PartialEq, Eq, PartialOrd, Ord, Debug, Copy, Clone)]
-pub struct Single(pub Card);
+pub struct Single(Card);
 
 impl Single {
     /** Create a new single from a card `c`.  Will return None if a Single