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/NonStupidBitfield.h

@@ -29,6 +29,8 @@
 // like std::bitset but less stupid and optimised for 
 // our use case (keeping track of memory invalidations)
 
+namespace melonDS
+{
 template <u32 Size>
 struct NonStupidBitField
 {
@@ -203,5 +205,6 @@ struct NonStupidBitField
     }
 };
 
+}
 
 #endif