GeckoCode: Cleanup

The active codes vector cannot safely be used outside the mutex,
move the lock out into RunCodeHandler. s_code_handler_installed was
also racing against SetActiveCodes since it's being written both
inside and outside the lock.

General cleanup. Add s_ prefixes, use constexpr, remove C casts.

Source/Core/Core/GeckoCode.h

@@ -34,7 +34,6 @@ public:
 };
 
 void SetActiveCodes(const std::vector<GeckoCode>& gcodes);
-bool RunActiveCodes();
 void RunCodeHandler();
 
 }  // namespace Gecko