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/Savestate.cpp

@@ -22,6 +22,8 @@
 #include "Savestate.h"
 #include "Platform.h"
 
+namespace melonDS
+{
 using Platform::Log;
 using Platform::LogLevel;
 
@@ -379,3 +381,5 @@ u32 Savestate::FindSection(const char* magic) const
     Log(LogLevel::Error, "savestate: section %s not found. blarg\n", magic);
     return NO_SECTION;
 }
+
+}