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/ARMJIT_x64/ARMJIT_GenOffsets.cpp

@@ -17,7 +17,7 @@
 */
 
 #include "../ARM.h"
-
+using namespace melonDS;
 int main(int argc, char* argv[])
 {
     FILE* f = fopen("ARMJIT_Offsets.h", "w");