Piece: don't take reference to self

src/repr/board.rs

@@ -79,10 +79,7 @@ impl fmt::Display for Board {
                 write!(
                     f,
                     "{}|",
-                    self.get((i, j).into())
+                    self.get((i, j).into()).map(Piece::symbol).unwrap_or(' ')
-                        .as_ref()
-                        .map(Piece::symbol)
-                        .unwrap_or(' ')
                 )?;
             }
             writeln!(f)?;

src/repr/piece.rs

@@ -7,21 +7,21 @@ pub enum Piece {
 }
 
 impl Piece {
-    pub const fn flip(&self) -> Self {
+    pub const fn flip(self) -> Self {
         match self {
             Piece::Black => Piece::White,
             Piece::White => Piece::Black,
         }
     }
 
-    pub const fn symbol(&self) -> char {
+    pub const fn symbol(self) -> char {
         match self {
             Piece::White => '■',
             Piece::Black => '□',
         }
     }
 
-    pub const fn text(&self) -> &'static str {
+    pub const fn text(self) -> &'static str {
         match self {
             Piece::Black => "Black",
             Piece::White => "White",