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_NAND.cpp

@@ -30,9 +30,9 @@
 
 #include "fatfs/ff.h"
 
-using namespace Platform;
+using namespace melonDS::Platform;
 
-namespace DSi_NAND
+namespace melonDS::DSi_NAND
 {
 
 NANDImage::NANDImage(Platform::FileHandle* nandfile, const DSiKey& es_keyY) noexcept : NANDImage(nandfile, es_keyY.data())