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/TinyVector.h

@@ -97,7 +97,7 @@ struct __attribute__((packed)) TinyVector
             Data[i] = Data[i + 1];*/
     }
 
-    int Find(T needle)
+    int Find(T needle) const
     {
         for (int i = 0; i < Length; i++)
         {
@@ -125,6 +125,12 @@ struct __attribute__((packed)) TinyVector
         assert(index >= 0 && index < Length);
         return Data[index];
     }
+
+    const T& operator[](int index) const
+    {
+        assert(index >= 0 && index < Length);
+        return Data[index];
+    }
 };
 }