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_I2C.h

@@ -26,11 +26,11 @@ namespace melonDS
 {
 class DSi_I2CHost;
 class DSi_Camera;
-
+class DSi;
 class DSi_I2CDevice
 {
 public:
-    DSi_I2CDevice(DSi_I2CHost* host) : Host(host) {}
+    DSi_I2CDevice(melonDS::DSi& dsi, DSi_I2CHost* host) : DSi(dsi), Host(host) {}
     virtual ~DSi_I2CDevice() {}
     virtual void Reset() = 0;
     virtual void DoSavestate(Savestate* file) = 0;
@@ -40,6 +40,7 @@ public:
     virtual void Write(u8 val, bool last) = 0;
 
 protected:
+    melonDS::DSi& DSi;
     DSi_I2CHost* Host;
 };
 
@@ -80,7 +81,7 @@ public:
         IRQ_ValidMask           = 0x7B,
     };
 
-    DSi_BPTWL(DSi_I2CHost* host);
+    DSi_BPTWL(melonDS::DSi& dsi, DSi_I2CHost* host);
     ~DSi_BPTWL() override;
     void Reset() override;
     void DoSavestate(Savestate* file) override;
@@ -153,7 +154,7 @@ private:
 class DSi_I2CHost
 {
 public:
-    DSi_I2CHost();
+    DSi_I2CHost(melonDS::DSi& dsi);
     ~DSi_I2CHost();
     void Reset();
     void DoSavestate(Savestate* file);
@@ -169,6 +170,7 @@ public:
     void WriteData(u8 val);
 
 private:
+    melonDS::DSi& DSi;
     u8 Cnt;
     u8 Data;