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/debug/GdbStub.h

@@ -13,6 +13,7 @@
 namespace Gdb
 {
 
+using namespace melonDS;
 enum class TgtStatus
 {
 	NoEvent,