API exposes memory layout of some runtime objects

[steve-s]

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

[vstinner]

This topic was covered by https://peps.python.org/pep-0620/ but this PEP scope was too broad and so it wasn't possible to implement it in a single Python versions or accept it since it is made of many non-trivial sub-issues.

[gvanrossum]

See also #45.

[encukou]

Issue for proposed guidelines: capi-workgroup/api-evolution#27