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.cpp

@@ -27,6 +27,8 @@
 #include "DSi_SPI_TSC.h"
 #include "Platform.h"
 
+namespace melonDS
+{
 using namespace Platform;
 
 
@@ -641,3 +643,5 @@ void SPIHost::WriteData(u8 val)
     u32 delay = 8 * (8 << (Cnt & 0x3));
     NDS::ScheduleEvent(NDS::Event_SPITransfer, false, delay, 0, 0);
 }
+
+}