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/FATStorage.cpp

@@ -25,6 +25,8 @@
 #include "FATStorage.h"
 #include "Platform.h"
 
+namespace melonDS
+{
 namespace fs = std::filesystem;
 using namespace Platform;
 
@@ -1104,3 +1106,5 @@ bool FATStorage::Save()
 
     return true;
 }
+
+}