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/Wifi.h

@@ -21,6 +21,8 @@
 
 #include "Savestate.h"
 
+namespace melonDS
+{
 class WifiAP;
 
 class Wifi
@@ -285,4 +287,5 @@ private:
     void RFTransfer_Type3();
 };
 
+}
 #endif