text: Title: Consider: dropping custom SaveLoad in favor of Pickle v.5 or joblib.dump? 
  Body: [PEP 574](https://www.python.org/dev/peps/pep-0574/) defines pickle-version-5, which supports alternate serialization of things like large numpy arrays. It's natively available in Python 3.8, with a PyPI backport [pickle5](https://pypi.org/project/pickle5/)  that works in 3.5, 3.6, and 3.7. 
  
  I'm not yet certain it allows the same mmap-on-load possible with `gensim.utils.SaveLoad`, but I suspect it might. 
  
  Similarly, `joblib`'s [dump()](https://joblib.readthedocs.io/en/latest/generated/joblib.dump.html) supports serializing large objects with large `numpy` arrays, and it is already what `sklearn` & others use for large models. Its matching [load()](https://joblib.readthedocs.io/en/latest/generated/joblib.load.html#joblib.load) has an `mmap_mode` option. 
  
  Gensim should consider whether either of these could be a superior alternative to `gensim.utils.SaveLoad` – in terms of being in sync with Python & related projects, & minimizing gensim idiosyncratic code. If so, a transition goal cold be set in some future release, with perhaps just one version supporting both modes (if continued forward-migration of old models is to be supported). 