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/ARMInterpreter_LoadStore.cpp

@@ -20,7 +20,7 @@
 #include "ARM.h"
 
 
-namespace ARMInterpreter
+namespace melonDS::ARMInterpreter
 {