gh-131238: Move _Py_VISIT_STACKREF() to pycore_stackref.h

[vstinner]

• Move _Py_VISIT_STACKREF() from pycore_gc.h to pycore_stackref.h.
• Remove pycore_interpframe.h include from pycore_genobject.h.
• Remove now useless includes from C files.
• Add pycore_interpframe_structs.h to Makefile.pre.in and pythoncore.vcxproj.

• Issue: Our internal headers are far too interlinked #131238

[colesbury]

This depends on _PyGC_VisitStackRef, which is in pycore_gc.h.

What's the motivation for moving the macro here?

[vstinner]

pycore_stackref.h depends on pycore_object.h which depends on pycore_gc.h.

_Py_VISIT_STACKREF() (currently defined in pycore_gc.h) uses PyStackRef_IsNull() which is defined in pycore_stackref.h.

Problem: If dependencies are made explicit (that's part of my work), pycore_gc.h should depend on pycore_stackref.h which creates a dependency cycle!

Moving _Py_VISIT_STACKREF() to pycore_stackref.h breaks this cycle.

[vstinner]

Previously, using _Py_VISIT_STACKREF() without including pycore_stackref.h worked thanks to luck and the fact that including one header pulled dozens of other headers. With my work on reducing dependencies, this luck is gone, and dependencies must be written explicitly.