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_Branch.cpp

@@ -19,12 +19,11 @@
 #include "ARM.h"
 #include "Platform.h"
 
+namespace melonDS::ARMInterpreter
+{
 using Platform::Log;
 using Platform::LogLevel;
 
-namespace ARMInterpreter
-{
-
 
 void A_B(ARM* cpu)
 {