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/DSi_I2C.cpp

@@ -26,6 +26,8 @@
 #include "SPI.h"
 #include "Platform.h"
 
+namespace melonDS
+{
 using Platform::Log;
 using Platform::LogLevel;
 
@@ -592,3 +594,5 @@ void DSi_I2CHost::WriteData(u8 val)
 {
     Data = val;
 }
+
+}