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/GPU.h

@@ -67,7 +67,7 @@ struct RenderSettings
 class GPU
 {
 public:
-    GPU(ARMJIT& jit) noexcept;
+    GPU(melonDS::NDS& nds) noexcept;
     ~GPU() noexcept;
     void Reset() noexcept;
     void Stop() noexcept;
@@ -536,7 +536,7 @@ public:
 
     void SyncDirtyFlags() noexcept;
 
-    ARMJIT& JIT;
+    melonDS::NDS& NDS;
     u16 VCount = 0;
     u16 TotalScanlines = 0;
     u16 DispStat[2] {};
@@ -582,7 +582,7 @@ public:
 
     GPU2D::Unit GPU2D_A;
     GPU2D::Unit GPU2D_B;
-    melonDS::GPU3D GPU3D {};
+    melonDS::GPU3D GPU3D;
 
     NonStupidBitField<128*1024/VRAMDirtyGranularity> VRAMDirty[9] {};
     VRAMTrackingSet<512*1024, 16*1024> VRAMDirty_ABG {};