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/ARCodeFile.cpp

@@ -21,6 +21,8 @@
 #include "ARCodeFile.h"
 #include "Platform.h"
 
+namespace melonDS
+{
 using namespace Platform;
 
 // TODO: import codes from other sources (usrcheat.dat, ...)
@@ -182,3 +184,5 @@ bool ARCodeFile::Save()
     CloseFile(f);
     return true;
 }
+
+}