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_SD.h

@@ -24,6 +24,8 @@
 #include "FATStorage.h"
 #include "Savestate.h"
 
+namespace melonDS
+{
 namespace DSi_NAND
 {
     class NANDImage;
@@ -169,4 +171,5 @@ private:
     u32 WriteBlock(u64 addr);
 };
 
+}
 #endif // DSI_SD_H