I decided to work on this issue while trying to convert the _PyObject_GC_TRACK() macro into a "static inline" function. Currently, the macro uses _PyGC_generation0 which is defined earlier by "extern PyGC_Head *_PyGC_generation0;".

Problem: _PyGC_generation0 no longer exists... Include/internal/mem.h now defines: "#define _PyGC_generation0 _PyRuntime.gc.generation0".

Include/internal/mem.h includes Include/objimpl.h, but Include/objimpl.h requires Include/internal/mem.h. The include order matters here, many header files are inter-dependent, and have two header files with the same name in Include/ and Include/internal/ causes issues depending where the #include is done...

My PR renames mem.h to pycore_objimpl.h and include pycore_objimpl.h at "the right place" in objimpl.h. Since objimpl.h controls where pycore_objimpl.h is imported, it's simpler to handle inter-dependencies simpler.

Some inter-dependencies issues are hidden by the fact the C macros don't really require functions, macros and variables in the right order. They "just work". But converting C macros to proper "static inline" functions expose many issues.