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.cpp

@@ -41,6 +41,8 @@
 
 #include "tiny-AES-c/aes.hpp"
 
+namespace melonDS
+{
 using namespace Platform;
 
 namespace DSi
@@ -3143,3 +3145,5 @@ void ARM7IOWrite32(u32 addr, u32 val)
 }
 
 }
+
+}