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/GPU3D.h

@@ -81,11 +81,12 @@ struct Polygon
 };
 
 class Renderer3D;
+class NDS;
 
 class GPU3D
 {
 public:
-    GPU3D() noexcept = default;
+    GPU3D(melonDS::NDS& nds) noexcept;
     ~GPU3D() noexcept = default;
     void Reset() noexcept;
 
@@ -124,6 +125,7 @@ public:
     void Write16(u32 addr, u16 val) noexcept;
     void Write32(u32 addr, u32 val) noexcept;
 private:
+    melonDS::NDS& NDS;
     typedef union
     {
         u64 _contents;