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/ARMJIT_A64/ARMJIT_Compiler.h

@@ -96,7 +96,11 @@ class Compiler : public Arm64Gen::ARM64XEmitter
 public:
     typedef void (Compiler::*CompileFunc)();
 
-    Compiler(ARMJIT& jit);
+#ifdef JIT_ENABLED
+    explicit Compiler(melonDS::NDS& nds);
+#else
+    explicit Compiler(melonDS::NDS& nds) : XEmitter(), NDS(nds) {}
+#endif
     ~Compiler();
 
     void PushRegs(bool saveHiRegs, bool saveRegsToBeChanged, bool allowUnload = true);
@@ -242,7 +246,7 @@ public:
         OtherCodeRegion = offset;
     }
 
-    ARMJIT& JIT;
+    melonDS::NDS& NDS;
     ptrdiff_t OtherCodeRegion;
 
     bool Exit;