[K/N] Refactor allocator modules ^KT-60928

- Move std_alloc, opt_alloc and custom_alloc into alloc/std,
  alloc/mimalloc and alloc/custom to mirror convention of gc,
  gcScheduler modules.
- Add alloc/common with common allocator API
- Add alloc/legacy with common implementation details of alloc/std and
  alloc/mimalloc. alloc/custom does not depend on alloc/legacy.
- Removes experimental_memory_manager_custom as it's now unused
- Additionally, renames experimental_memory_manager module into mm

kotlin-native/runtime/src/main/cpp/Runtime.cpp

@@ -11,7 +11,6 @@
 #include "MainQueueProcessor.hpp"
 #include "Memory.h"
 #include "ObjCExportInit.h"
-#include "ObjectAlloc.hpp"
 #include "Porting.h"
 #include "Runtime.h"
 #include "RuntimePrivate.hpp"