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.h

@@ -19,6 +19,8 @@
 #ifndef ARMINTERPRETER_BRANCH_H
 #define ARMINTERPRETER_BRANCH_H
 
+namespace melonDS
+{
 namespace ARMInterpreter
 {
 
@@ -36,4 +38,5 @@ void T_BL_LONG_2(ARM* cpu);
 
 }
 
+}
 #endif