Refactor NDS and DSi to be objects (#1893)

* First crack at refactoring NDS and DSi into objects

- Remove all global/`static` variables in `NDS` and related classes
- Rely more on virtual dispatch when we need to pick methods at runtime
- Pass `NDS&` or `DSi&` to its constituent components where necessary
- Introduce some headers or move some definitions to break `#include` cycles

* Refactor the frontend to accommodate the core's changes

* Move up `SchedList`'s declaration

- Move it to before the components are initialized so the `map`s inside are initialized
- Fields in C++ are initialized in the order they're declared

* Fix a crash when allocating memory

* Fix JIT-free builds

* Fix GDB-free builds

* Fix Linux builds

- Explicitly qualify some member types in NDS, since they share the same name as their classes

* Remove an unnecessary template argument

- This was causing the build to fail on macOS

* Fix ARM and Android builds

* Rename `Constants.h` to `MemConstants.h`

* Add `NDS::IsRunning()`

* Use an `#include` guard instead of `#pragma once`

src/DSi_Camera.h

@@ -25,17 +25,18 @@
 
 namespace melonDS
 {
+class DSi;
 class DSi_CamModule;
 
 class DSi_Camera : public DSi_I2CDevice
 {
 public:
-    DSi_Camera(DSi_I2CHost* host, u32 num);
+    DSi_Camera(melonDS::DSi& dsi, DSi_I2CHost* host, u32 num);
     ~DSi_Camera();
 
-    void DoSavestate(Savestate* file);
+    void DoSavestate(Savestate* file) override;
 
-    void Reset();
+    void Reset() override;
     void Stop();
     bool IsActivated();
 
@@ -45,9 +46,9 @@ public:
     // lengths in words
     int TransferScanline(u32* buffer, int maxlen);
 
-    void Acquire();
-    u8 Read(bool last);
-    void Write(u8 val, bool last);
+    void Acquire() override;
+    u8 Read(bool last) override;
+    void Write(u8 val, bool last) override;
 
     void InputFrame(u32* data, int width, int height, bool rgb);
 
@@ -84,7 +85,7 @@ private:
 class DSi_CamModule
 {
 public:
-    DSi_CamModule();
+    DSi_CamModule(melonDS::DSi& dsi);
     ~DSi_CamModule();
     void Reset();
     void Stop();
@@ -105,6 +106,7 @@ public:
     void Write32(u32 addr, u32 val);
 
 private:
+    melonDS::DSi& DSi;
     DSi_Camera* Camera0; // 78 / facing outside
     DSi_Camera* Camera1; // 7A / selfie cam