This commit adds `sb()->create_object()`, `sb()->get_object()`,
`sb()->check_object_type()` and `sb()->destroy_object()` in libretro
builds to keep track of all C++ objects allocated by the bindings in
libretro builds. This has some benefits:
* Any C++ objects allocated by the bindings that are still alive when
the game terminates can now be deallocated instead of being leaked
like before.
* We now keep track of the types of all objects allocated by the
bindings, so we will be able to detect when the bindings attempt to
access objects of mismatching type.
* Keeping track of all allocated objects is required to implement
libretro save states.
* Objects are now kept track of using numeric keys whose sizes are the
same on every platform rather than pointers, which helps with making
save states portable across platforms.
