text: Title: Keep "lifecycle log" in models
  Body: ## Why
  
  When Gensim users report issues with models, we have trouble deciding what they actually did, what version they used, etc. They're often confused themselves.
  
  This leads to protracted communication => increased support effort => slower issue resolution and more annoyance all around. Not good.
  
  ## What
  
  Keep an internal "lifecycle log" inside each model. This could be as simple as an internal list-of-strings attribute, e.g. `word2vec.lifecycle = ["init() created on 2020-06-20 15:49:03 UTC with Gensim 3.8.3 on Python 3.6", "train() called on 2020-06-20 15:49:04 UTC", "load() called on 2020-06-29 8:7:58 UTC", …]`. We would serialize this log on `save()`, and ask users to provide its value when investigating an issue.
  
  We probably want to keep this simple and human-readable, both in the API and log content. Not to over-engineer to start with – not attempting a full model recreation from the log or anything, I can see how this could turn into a rabbit hole.
  
  *Originally conceived by @gojomo in https://github.com/RaRe-Technologies/gensim/pull/2698#issuecomment-646929210.*