bpo-39542: Make _Py_NewReference() opaque in C API (GH-18346)

_Py_NewReference() becomes a regular opaque function, rather than a
static inline function in the C API (object.h), to better hide
implementation details.

Move _Py_tracemalloc_config from public pymem.h to internal
pycore_pymem.h header.

Make _Py_AddToAllObjects() private.