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/GPU3D.cpp

@@ -24,6 +24,8 @@
 #include "FIFO.h"
 #include "Platform.h"
 
+namespace melonDS
+{
 using Platform::Log;
 using Platform::LogLevel;
 
@@ -98,9 +100,6 @@ using Platform::LogLevel;
 // * additionally, some commands (BEGIN, LIGHT_VECTOR, BOXTEST) stall the polygon pipeline
 
 
-namespace GPU3D
-{
-
 const u8 CmdNumParams[256] =
 {
     // 0x00