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/Savestate.h

@@ -27,6 +27,8 @@
 #define SAVESTATE_MAJOR 11
 #define SAVESTATE_MINOR 1
 
+namespace melonDS
+{
 class Savestate
 {
 public:
@@ -117,5 +119,6 @@ private:
     bool buffer_owned;
     bool finished;
 };
+}
 
 #endif // SAVESTATE_H