Card's methods take values, not references

I expect that we will not want to retain the original copy of a Card
after merging, cutting, rotating, etc. This function signature says that
the value moves into the method and goes away. If this becomes an
annoyance, then we change it.

src/card.rs

@@ -115,19 +115,19 @@ impl Card {
   }
 
   /// Cuts this Card on the given line. Returns two cards
-  pub fn cut(&self, line: CutLine) -> (Self, Self) {
+  pub fn cut(self, line: CutLine) -> (Self, Self) {
     todo!();
   }
 
-  pub fn flip(&self, flip: FlipDir) -> (Self) {
+  pub fn flip(self, flip: FlipDir) -> Self {
     todo!();
   }
 
-  pub fn transpose(&self, other: &Self, selection: TransposeSelection) -> (Self, Self) {
+  pub fn transpose(self, other: Self, selection: TransposeSelection) -> (Self, Self) {
     todo!();
   }
 
-  pub fn rotate(&self, dir: RotationDir) {
+  pub fn rotate(self, dir: RotationDir) {
     todo!();
   }
 }