Implement Knight

rs/src/ui.rs

@@ -16,6 +16,7 @@ impl std::fmt::Display for board::PieceType {
     fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
         match self {
             board::PieceType::Pawn => write!(f, "P"),
+            board::PieceType::Knight => write!(f, "N"),
         }
     }
 }