Allow AREngine to be used independently of ARCodeFile (#2108)

* Make `EmuInstance::cheatFile` use a `unique_ptr`

- Fixes a memory leak, as the cheat file wasn't cleaned up in the destructor

* Split `AREngine` and `ARCodeFile` apart

- Suitable for frontends that have their own way of storing cheats
- Store the cheats in `AREngine` in a `std::vector`
- Apparently cheats are _supposed_ to be executed each frame; I didn't understand this until recently

src/ARCodeFile.h

@@ -48,14 +48,16 @@ class ARCodeFile
 {
 public:
     ARCodeFile(const std::string& filename);
-    ~ARCodeFile();
+    ~ARCodeFile() noexcept = default;
 
-    bool Error;
+    [[nodiscard]] std::vector<ARCode> GetCodes() const noexcept;
+
+    bool Error = false;
 
     bool Load();
     bool Save();
 
-    ARCodeCatList Categories;
+    ARCodeCatList Categories {};
 
 private:
     std::string Filename;