Autogen: separate API and ABI

[fangerer]

I’m running more and more into the situation where we want to add user-visible API functions (e.g. HPyCapsule_GetContext ind PR #308 ) but map them to a more generic context function (e.g. ctx_Capsule_Get).
The current autogen tool does not directly allow to do so. The more generic context function will also be exposed as API function.

I’m thinking of having a public_api.h and a context_abi.h file. Then it would be possible to define an (optional) mapping between API and ABI functions in, for instance, conf.py. The default behavior would still be that a function in public_api.h would also define an ABI function if no custom mapping was specified.

Reasons for my request are:

• In the docs, we state that the full API is defined in public_api.h but that’s actually no longer true. We already have macros/functions like HPy_New / _HPy_New.
• We can write custom API functions in misc.h / misc_trampolines.h (which don’t show up in public_api.h , ofc) but we still need to define something in public_api.h which will be both API/ABI but sometimes this function isn’t meant to be used directly but is still exposed.
• No debug wrapper will be generated for custom trampolines in misc.h / misc_trampolines.h.