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_NWifi.cpp

@@ -24,6 +24,9 @@
 #include "WifiAP.h"
 #include "Platform.h"
 
+namespace melonDS
+{
+
 using Platform::Log;
 using Platform::LogLevel;
 
@@ -1607,3 +1610,5 @@ void DSi_NWifi::MSTimer(u32 param)
 
     NDS::ScheduleEvent(NDS::Event_DSi_NWifi, true, 33611, 0, 0);
 }
+
+}