Cherry-picking one specific issue pointed out in #4.

Even the stable ABI still exposes few fields in the ABI, such as ob_refcnt. This is a problem:

• For alternative implementations, because their representations of those runtime objects may be completely different.
• For CPython, because it cannot change/optimize the memory layout.
• For extension authors: if the "stable" ABI needs to change, because of memory layout change, they need to rebuild wheels
• For users: they need to download new wheels, if extension authors don't build wheels for the new ABI, they need to do it themselves