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.
In big-endian libretro builds, the WebAssembly memory is reversed, so no
byte-swapping is required to read from/write to WebAssembly memory
(which is little-endian).
However, that means the ways to get and set values in WebAssembly memory
are endianness-dependent, so I've added the correct such ways for
big-endian platforms.
The binding coroutines in libretro builds are constructed on the VM
stack, so reallocating the VM memory would corrupt the memory of any
currently existing coroutines.
I've changed it so that the coroutines are no longer constructed on the
VM stack so that they're unaffected by VM memory reallocations, and
added a "slot" mechanism for storing variables on the VM stack. (Any
Ruby `VALUE`s used by a coroutine have to be stored on the VM stack so
that the Ruby garbage collector doesn't free them while they're being
used, which is why the slot mechanism is necessary.)
