Fix broken healing potions

We used `let Some(mut fighter) = fighter` instead of `let Some(ref mut
fighter) = fighter`.

Since the Fighter struct is a copy, this caused healing via potions to
not work.

doc/part-8-items.adoc

@@ -457,7 +457,7 @@ since it will probably be used multiple times. The constant
 ----
 /// heal by the given amount, without going over the maximum
 pub fn heal(&mut self, amount: i32) {
-    if let Some(mut fighter) = self.fighter {
+    if let Some(ref mut fighter) = self.fighter {
         fighter.hp += amount;
         if fighter.hp > fighter.max_hp {
             fighter.hp = fighter.max_hp;

src/bin/part-10-menu-saving.rs

@@ -206,7 +206,7 @@ impl Object {
 
     /// heal by the given amount, without going over the maximum
     pub fn heal(&mut self, amount: i32) {
-        if let Some(mut fighter) = self.fighter {
+        if let Some(ref mut fighter) = self.fighter {
             fighter.hp += amount;
             if fighter.hp > fighter.max_hp {
                 fighter.hp = fighter.max_hp;

src/bin/part-11-dungeon-progression.rs

@@ -221,7 +221,7 @@ impl Object {
 
     /// heal by the given amount, without going over the maximum
     pub fn heal(&mut self, amount: i32) {
-        if let Some(mut fighter) = self.fighter {
+        if let Some(ref mut fighter) = self.fighter {
             fighter.hp += amount;
             if fighter.hp > fighter.max_hp {
                 fighter.hp = fighter.max_hp;

src/bin/part-12-monster-item-progression.rs

@@ -219,7 +219,7 @@ impl Object {
 
     /// heal by the given amount, without going over the maximum
     pub fn heal(&mut self, amount: i32) {
-        if let Some(mut fighter) = self.fighter {
+        if let Some(ref mut fighter) = self.fighter {
             fighter.hp += amount;
             if fighter.hp > fighter.max_hp {
                 fighter.hp = fighter.max_hp;

src/bin/part-13-adventure-gear.rs

@@ -222,7 +222,7 @@ impl Object {
     /// heal by the given amount, without going over the maximum
     pub fn heal(&mut self, amount: i32, game: &Game) {
         let max_hp = self.max_hp(game);
-        if let Some(mut fighter) = self.fighter {
+        if let Some(ref mut fighter) = self.fighter {
             fighter.hp += amount;
             if fighter.hp > max_hp {
                 fighter.hp = max_hp;

src/bin/part-8-items.rs

@@ -191,7 +191,7 @@ impl Object {
 
     /// heal by the given amount, without going over the maximum
     pub fn heal(&mut self, amount: i32) {
-        if let Some(mut fighter) = self.fighter {
+        if let Some(ref mut fighter) = self.fighter {
             fighter.hp += amount;
             if fighter.hp > fighter.max_hp {
                 fighter.hp = fighter.max_hp;

src/bin/part-9-spells.rs

@@ -203,7 +203,7 @@ impl Object {
 
     /// heal by the given amount, without going over the maximum
     pub fn heal(&mut self, amount: i32) {
-        if let Some(mut fighter) = self.fighter {
+        if let Some(ref mut fighter) = self.fighter {
             fighter.hp += amount;
             if fighter.hp > fighter.max_hp {
                 fighter.hp = fighter.max_hp;