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/SPU.cpp

@@ -24,6 +24,8 @@
 #include "DSi.h"
 #include "SPU.h"
 
+namespace melonDS
+{
 using Platform::Log;
 using Platform::LogLevel;
 
@@ -1208,3 +1210,5 @@ void SPU::Write32(u32 addr, u32 val)
         }
     }
 }
+
+}