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_SD.cpp

@@ -24,6 +24,8 @@
 #include "DSi_NWifi.h"
 #include "Platform.h"
 
+namespace melonDS
+{
 using namespace Platform;
 
 // observed IRQ behavior during transfers
@@ -1095,3 +1097,5 @@ u32 DSi_MMCStorage::WriteBlock(u64 addr)
 
     return len;
 }
+
+}