Update function to borrow self, not take ownership

As @petertseng pointed out, I can update these functions to borrow
`self` if I give Direction and Position `Copy`

exercism#146 (comment)

Author: whit0694

exercises/robot-simulator/example.rs

@@ -1,4 +1,4 @@
-#[derive(PartialEq, Debug, Clone)]
+#[derive(PartialEq, Debug, Copy, Clone)]
 pub enum Direction {
     North,
     East,
@@ -26,6 +26,7 @@ impl Direction {
     }
 }
 
+#[derive(Clone, Copy)]
 struct Position {
     x: i32,
     y: i32,
@@ -36,7 +37,7 @@ impl Position {
         Position { x: x, y: y }
     }
 
-    fn advance(self, direction: &Direction) -> Self {
+    fn advance(&self, direction: &Direction) -> Self {
         match direction {
             &Direction::North => Self::new(self.x, self.y + 1),
             &Direction::South => Self::new(self.x, self.y - 1),
@@ -63,19 +64,19 @@ impl Robot {
         }
     }
 
-    pub fn turn_right(self) -> Self {
+    pub fn turn_right(&self) -> Self {
         Self::build(self.position, self.direction.next_clockwise())
     }
 
-    pub fn turn_left(self) -> Self {
+    pub fn turn_left(&self) -> Self {
         Self::build(self.position, self.direction.previous_clockwise())
     }
 
-    pub fn advance(self) -> Self {
+    pub fn advance(&self) -> Self {
         Self::build(self.position.advance(&self.direction), self.direction)
     }
 
-    pub fn instructions(self, instructions: &str) -> Self {
+    pub fn instructions(&self, instructions: &str) -> Self {
         let mut r = Self::build(self.position, self.direction);
         for c in instructions.chars() {
             r = r.execute(c);