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_NWifi.h

@@ -28,7 +28,7 @@ namespace melonDS
 class DSi_NWifi : public DSi_SDDevice
 {
 public:
-    DSi_NWifi(DSi_SDHost* host);
+    DSi_NWifi(melonDS::DSi& dsi, DSi_SDHost* host);
     ~DSi_NWifi();
 
     void Reset();
@@ -45,6 +45,7 @@ public:
     void MSTimer(u32 param);
 
 private:
+    melonDS::DSi& DSi;
     u32 TransferCmd;
     u32 TransferAddr;
     u32 RemSize;