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/WifiAP.cpp

@@ -27,6 +27,8 @@
 #include <stddef.h>
 #endif
 
+namespace melonDS
+{
 using Platform::Log;
 using Platform::LogLevel;
 
@@ -401,3 +403,5 @@ int WifiAP::RecvPacket(u8* data)
 
     return 0;
 }
+
+}