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_SD.h

@@ -32,12 +32,13 @@ namespace DSi_NAND
 }
 
 class DSi_SDDevice;
+class DSi;
 
 
 class DSi_SDHost
 {
 public:
-    DSi_SDHost(u32 num);
+    DSi_SDHost(melonDS::DSi& dsi, u32 num);
     ~DSi_SDHost();
 
     void CloseHandles();
@@ -69,6 +70,7 @@ public:
     void CheckSwapFIFO();
 
 private:
+    melonDS::DSi& DSi;
     u32 Num;
 
     u16 PortSelect;