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_DSP.h

@@ -27,6 +27,8 @@
 
 namespace Teakra { class Teakra; }
 
+namespace melonDS
+{
 class DSi_DSP
 {
 public:
@@ -104,5 +106,6 @@ private:
     u16 PDataDMAReadMMIO();
 };
 
+}
 #endif // DSI_DSP_H