Huge refactor: Make Core a singleton

src/backend/GameDB.cpp

@@ -1,8 +1,9 @@
 #include <GameDB.hpp>
-#include <Mem.hpp>
+#include <Core.hpp>
 
 namespace n64 {
-void GameDB::match(Mem &mem) {
+void GameDB::match() {
+  n64::Mem& mem = n64::Core::GetInstance().cpu->GetMem();
   const ROM &rom = mem.rom;
   for (const auto &[code, regions, saveType, name] : gamedb) {
     const bool matches_code = code == rom.code;