Piece: use char for symbol

src/board.rs

@@ -140,7 +140,7 @@ impl fmt::Display for Board {
                 write!(
                     f,
                     "{}|",
-                    self.get(i, j).as_ref().map(Piece::symbol).unwrap_or(" ")
+                    self.get(i, j).as_ref().map(Piece::symbol).unwrap_or(' ')
                 )?;
             }
             writeln!(f)?;

src/piece.rs

@@ -12,10 +12,10 @@ impl Piece {
         }
     }
 
-    pub const fn symbol(&self) -> &'static str {
+    pub const fn symbol(&self) -> char {
         match self {
-            Piece::White => "■",
-            Piece::Black => "□",
+            Piece::White => '■',
+            Piece::Black => '□',
         }
     }