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/GPU2D.h

@@ -22,10 +22,9 @@
 #include "types.h"
 #include "Savestate.h"
 
-namespace Melon
+namespace melonDS
 {
 class GPU;
-}
 
 namespace GPU2D
 {
@@ -35,7 +34,7 @@ class Unit
 public:
     // take a reference to the GPU so we can access its state
     // and ensure that it's not null
-    Unit(u32 num, Melon::GPU& gpu);
+    Unit(u32 num, melonDS::GPU& gpu);
 
     Unit(const Unit&) = delete;
     Unit& operator=(const Unit&) = delete;
@@ -124,7 +123,7 @@ public:
 
     u16 MasterBrightness;
 private:
-    Melon::GPU& GPU;
+    melonDS::GPU& GPU;
 };
 
 class Renderer2D
@@ -150,4 +149,5 @@ protected:
 
 }
 
+}
 #endif