Move all core types into namespaces (#1886)

* Reorganize namespaces

- Most types are now moved into the `melonDS` namespace
- Only good chance to do this for a while, since a big refactor is next

* Fix the build

src/DSi_Camera.cpp

@@ -23,6 +23,8 @@
 #include "DSi_Camera.h"
 #include "Platform.h"
 
+namespace melonDS
+{
 using Platform::Log;
 using Platform::LogLevel;
 
@@ -793,3 +795,5 @@ void DSi_Camera::InputFrame(u32* data, int width, int height, bool rgb)
         }
     }
 }
+
+}