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/RTC.cpp

@@ -21,6 +21,8 @@
 #include "RTC.h"
 #include "Platform.h"
 
+namespace melonDS
+{
 using Platform::Log;
 using Platform::LogLevel;
 
@@ -940,3 +942,5 @@ void RTC::Write(u16 val, bool byte)
     else
         IO = (IO & 0x0001) | (val & 0xFFFE);
 }
+
+}