Improve API/implementation for HPyCapsule_Destructor

[fangerer]

This resolved #377.
In this PR, I'm introducing macro HPyDef_CAPSULE_DESTRUCTOR (much like HPyDev_METH macro) and removing HPyCapsule_GetDestructor. With this, we can significantly simplify our capsule implementation because we never need to hand out the HPy capsule destructor function while the new macro will generate an appropriate CPython trampoline.

Removing HPyCapsule_GetDestructor should be fine since I didn't find a single usage of PyCapsule_GetDestructor in the C code of the top4000 packages.

[mattip]

LGTM, but will defer to @antocuni

[antocuni]

LGTM, apart the name of the macro: the idea of the various existing HPyDef_* macros is that each of them defines a static variable whose C type is actually HPyDef.

HPyDef_CAPSULE_DESTRUCTOR does something different, so it should be named differently; it belongs to the HPyCapsule "namespace", so what about HPyCapsule_DESTRUCTOR?

[fangerer]

LGTM, apart the name of the macro: the idea of the various existing HPyDef_* macros is that each of them defines a static variable whose C type is actually HPyDef.

HPyDef_CAPSULE_DESTRUCTOR does something different, so it should be named differently; it belongs to the HPyCapsule "namespace", so what about HPyCapsule_DESTRUCTOR?

Okay, I'm not objected to rename it. I did that for consistency but didn't have in mind that the HPyDef_* prefix is because of the HPyDef type.

[antocuni]

LGTM, thanks