Sprinkle const around where appropriate (#1909)

* Sprinkle `const` around where appropriate

- This will make it easier to use `NDS` objects in `const` contexts (e.g. `const` parameters or methods)

* Remove the `const` qualifier on `DSi_DSP::DSPRead16`

- MMIO reads can be non-pure, so this may not be `const` in the future

src/AREngine.h

@@ -33,7 +33,7 @@ public:
     void SetCodeFile(ARCodeFile* file) { CodeFile = file; }
 
     void RunCheats();
-    void RunCheat(ARCode& arcode);
+    void RunCheat(const ARCode& arcode);
 private:
     melonDS::NDS& NDS;
     ARCodeFile* CodeFile; // AR code file - frontend is responsible for managing this