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/SPI_Firmware.cpp

@@ -20,6 +20,8 @@
 #include "SPI.h"
 #include "Platform.h"
 
+namespace melonDS
+{
 using Platform::Log;
 using Platform::LogLevel;
 
@@ -404,3 +406,5 @@ void Firmware::UpdateChecksums()
         u.UpdateChecksum();
     }
 }
+
+}