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/RTC.h

@@ -48,7 +48,7 @@ public:
         u8 AlarmDate2[3];
     };
 
-    RTC();
+    RTC(melonDS::NDS& nds);
     ~RTC();
 
     void Reset();
@@ -66,9 +66,7 @@ public:
     void Write(u16 val, bool byte);
 
 private:
-    /// This value represents the Nintendo DS IO register,
-    /// \em not the value of the system's clock.
-    /// The actual system time is taken directly from the host.
+    melonDS::NDS& NDS;
     u16 IO;
 
     u8 Input;